1 Introduction
In this paper, we prove several results about basic invariants of a closed subscheme Z of a smooth, irreducible complex ndimensional algebraic variety X. We give a characterisation of the local cohomological dimension $\mathrm {lcd} (X, Z)$ in terms of coherent sheaf data associated to a log resolution of $(X, Z)$ , complementing the celebrated topological criterion in [Reference Ogus43]. We also obtain local vanishing results for sheaves of forms with log poles associated to such a resolution, generalising Nakanotype results in [Reference Saito52] and [Reference Mustaţă and Popa36]. We prove a vanishing result for cohomologies of the graded pieces of the Du Bois complex when Z is a local complete intersection, extending the study of higher Du Bois singularities of hypersurfaces in [Reference Mustaţă, Olano, Popa and Witaszek41] and [Reference Jung, Kim, Saito and Yoon24]. When Z has isolated singularities, we refine a result in [Reference Kebekus and Schnell25] on the coincidence of hdifferentials and reflexive differentials, for forms of low degree. As a byproduct, we also obtain new proofs of various results in the literature, for instance, an injectivity theorem in [Reference Kovács and Schwede29] or various cases of results related to local cohomology in [Reference Dao and Takagi8] and [Reference Ma, Schwede and Shimomoto33].
The common theme leading to the proof of all these results is the study of the Hodge filtration on local cohomology. The local cohomology sheaves of $\mathscr {O}_X$ along Z are important and subtle invariants of the pair $(X,Z)$ ; while they are not coherent over $\mathscr {O}_X$ , they are wellbehaved modules over the sheaf $\mathscr {D}_X$ of differential operators. Exploiting this fact has been the key to important developments, especially in commutative algebra, starting with the foundational paper of Lyubeznik [Reference Lyubeznik32]. Our focus in this paper is the fact that they have an even more refined structure, namely, that of mixed Hodge modules; as such, they come endowed with a canonical Hodge filtration. We study this filtration on local cohomology and relate it to various invariants of Z mentioned above.
Local cohomology sheaves as mixed Hodge modules. We consider the local cohomology sheaves ${\mathcal H}^q_Z(\mathscr {O}_X)$ , where q is a positive integer (for a review of these objects, see §2.2).Footnote ^{1} It is well understood that all ${\mathcal H}^q_Z(\mathscr {O}_X)$ carry the structure of (regular, holonomic) filtered $\mathscr {D}_X$ modules underlying mixed Hodge modules on X, with support in Z (see §2.3). In particular, they come endowed with a good filtration $F_k {\mathcal H}^q_Z(\mathscr {O}_X)$ by coherent subsheaves, with $k \ge 0$ , called the Hodge filtration. These data depend only on the reduced structure of Z.
When Z is a hypersurface, only ${\mathcal H}^1_Z(\mathscr {O}_X)$ is nontrivial, and, in fact,
where $\mathscr {O}_X (*Z)$ is the sheaf of rational functions on X with poles along Z. Hence, the study of the Hodge filtration on ${\mathcal H}^1_Z(\mathscr {O}_X)$ reduces to that of the Hodge filtration on $\mathscr {O}_X(*Z)$ , or equivalently, to that of the Hodge ideals treated in [Reference Mustaţă and Popa36]. As in that paper, for concrete applications, one essential point is to provide an alternative description of the Hodge filtration in terms of log resolutions. We discuss this next.
Suppose that $f\colon Y\to X$ is a log resolution of the pair $(X, Z)$ , assumed to be an isomorphism over the complement of Z in X. We denote $E = f^{1}(Z)_{\mathrm {red}}$ , which is a simple normal crossing (SNC) divisor on Y. We observe in §2.4 that there is a filtered complex of right $f^{1} \mathscr {D}_X$ modules
such that, restricting the discussion to $q\geq 2$ for simplicity, we have an isomorphism
Moreover, the Hodge filtration $F_\bullet {\mathcal H}^q_Z(\omega _X)$ is obtained as the image of the pushforward of a natural filtration $F_\bullet A^{\bullet }$ , also described in §2.4. This description parallels the birational definition of Hodge ideals of hypersurfaces in [Reference Mustaţă and Popa36].
Once this birational description has been established, the main engine towards applications is the strictness property of the Hodge filtration on direct images of Hodge modules via projective morphisms. This is a vast generalisation of the $E_1$ degeneration of the Hodgetode Rham spectral sequence, established by Saito [Reference Saito48], [Reference Saito49]. Its main consequence to local cohomology is described in Proposition 2.9; here, we start by mentioning the most immediate application, namely, an injectivity theorem.
Note first that, with the notation above, there is a natural morphism $\mathscr {O}_Z \to \mathbf {R} f_* \mathscr {O}_E$ in $\mathbf {D}^b \big (\mathrm {Coh} (X)\big )$ , which by duality gives rise to a morphism
where the notation refers to the respective dualising complexes (for E, we of course have $\omega _E^{\bullet } = \omega _E [n 1]$ ). On the other hand, there is a morphism
to the total (derived) local cohomology of $\omega _X$ , arising from the natural morphism $\mathbf {R} \mathcal {H} om_X (\mathscr {O}_Z, \omega _X) \to \mathbf {R} \underline {\Gamma _Z} (\omega _X)$ . Here, $\underline {\Gamma _Z} ()$ denotes the sheaf version of the functor of sections with support in Z.
Theorem A. The morphism obtained as the composition
is injective on cohomology, that is, the induced morphisms on cohomology give for each q an injection
Since the morphism in the theorem factors through $\omega _Z^{\bullet }$ via $\alpha $ , this recovers, in particular, the following very useful result of Kovács and Schwede:
Corollary B [Reference Kovács and Schwede29, Theorem 3.3]
For each i, the natural homomorphism
is injective.
As the authors explain in loc. cit., this can be thought of as a GrauertRiemenschneider type result. To be more explicit, it says that for each $q \ge 1$ , the natural morphism
is injective (in particular, if Z is CohenMacaulay of pure codimension r, then $R^q f_* \omega _E= 0$ for $q \neq r1$ ). We make use of this when studying the local cohomological dimension of Z in terms of depth.
For a hypersurface Z, one of the key tools in the study of the Hodge filtration $F_k \mathscr {O}_X(*Z)$ is its containment in the pole order filtration $P_k \mathscr {O}_X(*Z) = \mathscr {O}_X\big ((k+1)Z\big )$ , as noted in [Reference Saito50]. In arbitrary codimension, it is still the case that
for all k, where $O_k$ is an order filtration analogous to $P_k$ . Unless $q = \mathrm {codim}_X(Z)$ , however, work of Lyubeznik [Reference Lyubeznik32] implies that the sheaves $O_k{\mathcal H}^q_Z(\mathscr {O}_X)$ are not coherent. A natural replacement seems to be an Ext filtration defined as
and satisfying $E_k \subseteq O_k$ . Note that both $O_k{\mathcal H}^q_Z(\mathscr {O}_X)$ and $E_k{\mathcal H}^q_Z(\mathscr {O}_X)$ depend on the schemetheoretic structure of Z (we get the smallest version by taking Z to be reduced). When Z is a local complete intersection of pure codimension r, then the two filtrations on ${\mathcal H}^r_Z(\mathscr {O}_X)$ coincide; we show that, in this case, they also coincide with the Hodge filtration if and only if Z is smooth (see Corollary 3.26).
In general, Theorem A and the birational interpretation of the Hodge filtration in §2.4 imply that
for all q. Furthermore, as a combination of our results with a wellknown characterisation of Du Bois singularities (see [Reference Steenbrink59], [Reference Schwede55]), we obtain:
Theorem C. Let $Z \subseteq X$ be a closed reduced subscheme of codimension r. If Z is Du Bois, then
If we assume that Z is CohenMacaulay, of pure dimension, then $F_0{\mathcal H}^q_Z(\mathscr {O}_X) = 0$ for all $q \neq r$ and
It is a very interesting question whether $F_k \subseteq E_k$ for all $k \ge 1$ , as the difference between the two should be a subtle measure of the singularities of Z by analogy with the case of hypersurfaces. This does happen when Z is a local complete intersection, in which case, Theorem F provides a vast generalisation of the last equivalence in the theorem above. Under this assumption, we also expect that the equality $F_1 = E_1$ implies that Z has rational singularities and an even stronger statement regarding the equality $F_k = E_k$ for higher k (see Conjectures 3.20 and 3.31).
Local vanishing and local cohomological dimension. Since the ambient space X is smooth, it is well known that the lowest index q for which $\mathcal {H}^{q}_Z (\mathscr {O}_X)\neq 0$ is equal to the codimension of Z. On the other hand, the highest such index is a more mysterious and much studied invariant, the local cohomological dimension of Z in X:
Note that $\mathrm {lcd}(X, Z)$ is more commonly defined as the minimal integer q, such that ${\mathcal H}_Z^i(\mathscr {F}) = 0$ for all $i> q$ and all quasicoherent sheaves $\mathscr {F}$ on X, but the two definitions agree (see e.g. [Reference Ogus43, Proposition 2.1].) Our study of the Hodge filtration allows us to provide a new perspective on $\mathrm {lcd}(X, Z)$ , by relating it to invariants arising from log resolutions.
Note, to begin with, that thanks to the GrauertRiemenschneider theorem, we have
hence, we can alternatively think of Theorem A as a result about the higher direct images $R^q f_* \omega _Y (E)$ . For instance, they must vanish when $\mathcal {H}^{q+1}_Z (\mathscr {O}_X)= 0$ and $q \ge 1$ . Using the same circle of ideas, based on the strictness property and the birational description of the Hodge filtration, this can be extended to a Nakanotype vanishing result for the higher direct images of all bundles of forms with log poles along E.
Theorem D. Let $Z \subseteq X$ be a closed subscheme of codimension r, and let $c: = \mathrm {lcd}(X, Z)$ . If $f\colon Y \to X$ is a log resolution of $(X, Z)$ , which is an isomorphism away from Z, and $E = f^{1}(Z)_{\mathrm {red}}$ , then
in either of the following two cases:

1. $p+q\geq n+1$ and $q\leq r2$ ;

2. $p+q\geq n+c$ .
In the case when Z is a hypersurface (so that $c =1$ ), this is a result of Saito [Reference Saito52, Corollary 3] (cf. also [Reference Mustaţă and Popa36, Theorem 32.1]). We remark that the part of the statement saying that vanishing holds whenever $p + q \ge n + c$ can also be obtained as a consequence of Theorem E below. Various bounds on the local cohomological dimension that are relevant to this theorem can be found in §2.6. At least for local complete intersections, the range of vanishing in Theorem D could perhaps be further improved by analogy with [Reference Mustaţă and Popa39, Theorem D], though this will rely on connections with the BernsteinSato polynomial of Z not known at the moment (cf. Remark 3.30).
As one of the most important applications of the techniques in this paper, we go in the opposite direction and obtain a characterisation of the local cohomological dimension $\mathrm {lcd}(X, Z)$ in terms of the vanishing of sheaves of the form $R^q f_* \Omega _Y^p (\log E)$ associated to a log resolution.
Theorem E. Let Z be a closed subscheme of X and c a positive integer. Then the following are equivalent:

1. $\mathrm {lcd}(X, Z) \le c$ ;

2. For any (some) log resolution $f\colon Y\to X$ of the pair $(X, Z)$ , assumed to be an isomorphism over the complement of Z in X, if $E = f^{1}(Z)_{\mathrm {red}}$ , we have
$$ \begin{align*}R^{j + i} f_* \Omega_Y^{ni} (\log E) = 0, \,\,\,\,\,\,\mathrm{for ~all}\,\,\,\,j \ge c, \, i \ge 0.\end{align*} $$
Over our base field ${\mathbf C}$ , this provides an alternative algebraic criterion in terms of finitely many coherent sheaves, complementing Ogus’ celebrated topological criterion [Reference Ogus43, Theorem 2.13], and answering, in particular, a problem raised there. The equivalence between Ogus’ criterion and ours seems unclear at the moment and is an intriguing topic of study (see Remark 4.12 for further discussion). In §4.2, we also give concrete applications of this characterisation; more on this below as well.
Note: While throughout this paper we focus on the case of algebraic varieties, it is worth noting that the criterion in Theorem E holds in the analytic setting as well, that is, when Z is an analytic subspace of a complex manifold X. The same holds for Theorem D. This is due to the fact that our constructions and arguments based on the theory of mixed Hodge modules apply equally well in this setting (see Remarks 2.11 and 4.14).
The proof of the theorem relies on a simple strategy involving the Hodge filtration, namely, showing, under the appropriate hypotheses, that:

1. $F_0 {\mathcal H}^q_Z (\mathscr {O}_X) = 0$ and

2. $F_\bullet {\mathcal H}^q_Z (\mathscr {O}_X)$ is generated at level $0$ .
The second condition essentially means that the entire Hodge filtration is determined by the initial term $F_0 {\mathcal H}^q_Z (\mathscr {O}_X)$ , up to applying differential operators. The key technical tool is, therefore, a local vanishing criterion for the generation level of the Hodge filtration, in the style of [Reference Mustaţă and Popa36, Theorem 17.1] in the case of hypersurfaces. This is stated as Theorem 4.2 below.
Numerous works have studied bounds on the local cohomological dimension in terms of the depth of the local rings at points of Z. Theorem E leads to a unified approach to previously known such bounds (for example, some of the statements in [Reference Hartshorne16], [Reference Ogus43], [Reference Varbaro61], [Reference Dao and Takagi8]), as well as to new results. Among the latter, we show in Corollary 4.29 that if Z has quotient singularities and codimension r, then
Due to results of Ogus, this, in turn, implies that subvarieties $Z \subseteq {\mathbf P}^n$ with quotient singularities behave like local complete intersections in other respects as well, for instance, satisfying a BarthLefschetztype result (see Corollary 4.30). We refrain from including more material here (for further details and examples, see §4.2).
The Du Bois complex and differentials on singular spaces. Our results regarding the Hodge filtration on local cohomology, and perhaps somewhat surprisingly, the characterisation of local cohomological dimension in Theorem E, can be applied to the study of the Du Bois complex and of various types of differentials on a reduced closed subscheme $Z \subseteq X$ .
Recall that the Du Bois complex $\underline {\Omega }_Z^{\bullet }$ is an object in the derived category of filtered complexes on Z. The shifted graded pieces $\underline {\Omega }_Z^p:=\mathrm { Gr}^p_F\underline {\Omega }_Z^{\bullet }[p]$ are objects in the derived category of coherent sheaves on Z, playing a role similar to that of the bundles of holomorphic forms $\Omega _Z^p$ on a smooth Z. There are, in fact, canonical morphisms $\Omega _Z^p\to \underline {\Omega }_Z^p$ that are isomorphisms over the smooth locus of Z. By definition, the one for $p=0$ is an isomorphism precisely when Z has Du Bois singularities (see Chapter 5 for more details).
Following the terminology from [Reference Jung, Kim, Saito and Yoon24], we say that Z has only higher pDu Bois singularities if the canonical morphisms $\Omega _Z^k\to \underline {\Omega }_Z^k$ are isomorphisms for all $0\leq k\leq p$ . The first result concerning varieties with this property was obtained in [Reference Mustaţă, Olano, Popa and Witaszek41], where is was shown that if Z is a hypersurface whose minimal exponent is $\geq p+1$ , then Z has only higher pDu Bois singularities; recall that the minimal exponent of Z, which can be defined via the BernsteinSato polynomial of Z, roughly describes how close the Hodge filtration and pole order filtration are on the localisation $\mathscr {O}_X(*Z)$ . The converse to this result was obtained in [Reference Jung, Kim, Saito and Yoon24]. The Hodge filtration on local cohomology allows us to extend these results to all local complete intersections.
Concretely, if Z is reduced and a local complete intersection of pure codimension r, then the singularity level of the Hodge filtration on $\mathcal {H}^r_Z \mathscr {O}_X$ is
with the convention that $p (Z) = 1$ if there are no such k. We show that this invariant only depends on Z and not on its embedding in a smooth variety. It is easy to check that $p(Z) = \infty $ if and only if Z is smooth (see Corollary 3.26). In fact, we have the following explicit upper bound for singular Z (see Theorem 3.39):
By a result of Saito [Reference Saito53], if Z is a hypersurface in X, then $p(Z)=[ \widetilde {\alpha }(Z)]1$ , where $\widetilde {\alpha }(Z)$ is the minimal exponent of Z. We expect that, in general, $p(Z)$ can be described in terms of the BernsteinSato polynomial of Z studied in [Reference Budur, Mustaţǎ and Saito6] (see Conjecture 3.31 for the statement). An interpretation of $p(Z)$ in terms of the Hodge ideals associated to products of equations defining Z is given by Proposition 3.34, leading to restriction and semicontinuity results for this invariant (see Theorems 3.36 and 3.37). The proof of (1.2) makes use of this semicontinuity property of $p(Z)$ .
The following is our main result, relating $p(Z)$ to the behavior of the Du Bois complex of Z. The proof builds on the case of hypersurfaces, which, as already mentioned, is treated in [Reference Mustaţă, Olano, Popa and Witaszek41] and [Reference Jung, Kim, Saito and Yoon24].
Theorem F. If Z is a reduced, local complete intersection closed subscheme of the smooth, irreducible variety X, then for every nonnegative integer p, we have $p(Z)\geq p$ if and only if Z has only higher pDu Bois singularities.
We also prove a related result concerning the vanishing of individual cohomology sheaves $\mathcal {H}^i \underline {\Omega }_Z^p$ , with $i> 0$ , in terms of the size of the locus in Z, where $p(Z)<p$ (see Theorem 5.7 for the precise statement). A consequence is that if the singular locus of the local complete intersection Z has dimension s, then for all $p \ge 0$ we have
In a different direction, the criterion in Theorem E can be rephrased in terms of the Du Bois complex (see Corollary 5.3), thanks to a result of Steenbrink [Reference Steenbrink59]. This allows us to obtain in §5.4 results on differentials on the singular variety Z as consequences of bounds on $\mathrm {lcd}(X, Z)$ . We state here one result regarding hdifferentials; their theory is one of the possible approaches to differential forms on singular spaces, as explained in [Reference Huber and Jörder19], where it is also shown that they are isomorphic to $\mathcal {H}^0 \underline {\Omega }_Z^k$ .
In the ideal situation, hdifferentials coincide with the reflexive differentials $\Omega _Z^{[k]} : = (\Omega _Z^k)^{\vee \vee }$ , and a recent result of KebekusSchnell [Reference Kebekus and Schnell25, Corollary 1.12] states that this is indeed the case for all k if Z is a variety with rational singularities. We show the following improvement for low k when Z has isolated singularities, using, in addition, input from mixed Hodge theory:
Theorem G. If Z is a variety with isolated singularities and $\mathrm {depth} (\mathscr {O}_Z) \ge k +2$ , then the hdifferentials and reflexive differentials of degree k on Z coincide.
Further applications of results on local cohomological dimension to hdifferentials are obtained in §5.4, including a statement analogous to Theorem G but depending only on the codimension of Z.
There are numerous conjectures and open problems suggested by this work that are scattered throughout the text and that we believe are important for further developments. Here is an informal sample: the connection between the Hodge filtration on local cohomology and the BernsteinSato polynomial (and perhaps a version of the Vfiltration) for local complete intersections, including applications to rational singularities; the equivalence between our and Ogus’ characterisation of local cohomological dimension; further local vanishing with applications to local cohomological dimension and reflexive differentials.
Notation. We collect, here, some notation that appears throughout the paper, with references to where each item is described. Note that X will always be a smooth, irreducible complex variety of dimension n and Z a closed subscheme of X.

1. $\mathrm {lcd}(X, Z)$ , the local cohomological dimension of Z in X (see Formula (1.1));

2. $\mathscr {D}_X$ , the sheaf of differential operators on X;

3. ${\mathbf Q}_X^H[n]$ , the trivial pure Hodge module on X (see § 2.1);

4. $(\mathcal {M},F)(q)$ , the Tate twist by q of the filtered $\mathscr {D}_X$ module $(\mathcal {M},F)$ (see § 2.1);

5. $\mathrm {Gr}^F_k\mathrm {DR}_X(\mathcal {M}, F)$ , the $k^{\mathrm {th}}$ graded piece of the de Rham complex of the filtered $\mathscr {D}_X$ module $(\mathcal {M},F)$ (see § 2.1);

6. ${\mathcal H}^q_Z(\mathcal {M})$ , the $q^{\mathrm {th}}$ local cohomology of $\mathcal {M}$ with support in Z (see § 2.2);

7. $\mathscr {O}_X(*Z)$ , the sheaf of rational functions on X with poles along Z, when Z is a hypersurface (see Example 2.1);

8. $F_k{\mathcal H}^q_Z(\mathscr {O}_X)$ , the $k^{\mathrm {th}}$ term of the Hodge filtration on ${\mathcal H}^q_Z(\mathscr {O}_X)$ (see § 2.3);

9. $O_k{\mathcal H}^q_Z(\mathscr {O}_X)$ , the $k^{\mathrm {th}}$ term of the order filtration on ${\mathcal H}^q_Z(\mathscr {O}_X)$ (see Definition 3.3);

10. $E_k{\mathcal H}^q(\mathscr {O}_X)$ , the $k^{\mathrm {th}}$ term of the Ext filtration on ${\mathcal H}^q_Z(\mathscr {O}_X)$ (see Definition 3.6);

11. $p(Z)$ , the singularity level of the Hodge filtration on ${\mathcal H}^r_Z(\mathscr {O}_X)$ , when Z is a local complete intersection of codimension r (see Definition 3.27);

12. $\Omega _Z^p$ , the sheaf of pKähler differentials on Z;

13. $\Omega _Z^{[k]}=(\Omega _Z^k)^{\vee \vee }$ , the sheaf of reflexive pKähler differentials on Z;

14. $\underline {\Omega }_Z^p$ , the (shifted) $p^{\mathrm {th}}$ truncation of the Du Bois complex (see § 5).
2 Background and study of the Hodge filtration
2.1 $\mathscr {D}$ modules and mixed Hodge modules
Given a smooth, irreducible, ndimensional complex algebraic variety X, we denote by $\mathscr {D}_X$ the sheaf of differential operators on X. For basic facts in the theory of $\mathscr {D}_X$ modules, we refer to [Reference Hotta, Takeuchi and Tanisaki18].
We only recall here a couple of things: first, an exhaustive filtration $F_{\bullet }$ on a left $\mathscr {D}_X$ module $\mathcal {M}$ (always assumed to be compatible with the filtration $F_\bullet \mathscr {D}_X$ by the order of differential operators) is good if $F_p\mathcal {M}$ is coherent for all p and there is q, such that
In this case, we say that the filtration is generated at level q.
Next, there is an equivalence of categories between left and right $\mathscr {D}_X$ modules, such that, if $\mathcal {M}^r$ is the right $\mathscr {D}_X$ module corresponding to the left $\mathscr {D}_X$ module $\mathcal {M}$ , we have an isomorphism of underlying $\mathscr {O}_X$ modules $\mathcal {M}^r\simeq \omega _X\otimes _{\mathscr {O}_X}\mathcal {M}$ . We will mostly work with left $\mathscr {D}_X$ modules, but it will sometimes be convenient to work with their right counterparts. We note that if they are endowed with a good filtration as above, then the leftright rule for the filtration is by convention
For the basic notions and results on mixed Hodge modules, we refer to [Reference Saito48] and [Reference Saito49]. In what follows, we refer to a mixed Hodge module on X simply as a Hodge module. Recall that such an object consists of a $\mathscr {D}_X$ module on X (always assumed to be holonomic and with regular singularities), endowed with a good filtration called the Hodge filtration, and with several other pieces of data satisfying an involved set of conditions that we will not be directly concerned with in this paper. We will say that this filtered $\mathscr {D}_X$ module underlies the respective Hodge module. An important fact is that every morphism of Hodge modules is strict with respect to the Hodge filtrations (see also [Reference Mustaţă and Popa36, Chapter C] and [Reference Mustaţă and Popa37, §1] for further review).
An important example of (pure) Hodge module is the trivial Hodge module ${\mathbf Q}_X^H[n]$ . The underlying $\mathscr {D}_X$ module is $\mathscr {O}_X$ , while the Hodge filtration is defined by the condition $\mathrm {Gr}^F_p(\mathscr {O}_X)=0$ for all $p\neq 0$ .
Given a filtered $\mathscr {D}_X$ module $(\mathcal {M},F)$ and $q\in {\mathbf Z}$ , the Tate twist $(\mathcal {M},F)(q)$ is the filtered $\mathscr {D}_X$ module $\big (\mathcal {M}, F[q]\big )$ , where
For every left $\mathscr {D}_X$ module $\mathcal {M}$ , the de Rham complex $\mathrm {DR}_X(\mathcal {M})$ is the complex
placed in cohomological degrees $n,\ldots ,0$ . If $\mathcal {M}$ carries a good filtration F, the de Rham complex has an induced filtration, such that, for every integer k, the graded piece $\mathrm {Gr}^F_k\mathrm {DR}_X(\mathcal {M},F)$ is given by
where $\mathrm {Gr}^F_i \mathcal {M} =F_i\mathcal {M}/F_{i1}\mathcal {M}$ for all $i\in {\mathbf Z}$ . Note that this is a complex of coherent $\mathscr {O}_X$ modules. The filtered de Rham complex of a filtered right $\mathscr {D}_X$ module is the filtered de Rham complex of the corresponding left $\mathscr {D}_X$ module. When $(\mathcal {M}, F)$ underlies a Hodge module M, we sometimes use, alternatively, the notation $\mathrm {DR}_X(M)$ and $\mathrm {Gr}^F_p\mathrm {DR}_X(M)$ .
Since morphisms of Hodge modules are strict with respect to the Hodge filtration, $\mathrm {Gr}^F_k\mathrm {DR}_X()$ gives an exact functor from the abelian category of Hodge modules on X to the abelian category of bounded complexes of coherent sheaves on X. This induces an exact functor, also denoted $\mathrm {Gr}^F_k\mathrm {DR}_X()$ , from the derived category $\mathbf {D}^b\big (\mathrm {MHM}(X)\big )$ of Hodge modules on X to the derived category $\mathbf {D}^b\big (\mathrm {Coh}(X)\big )$ . By considering the truncation functors associated to the standard tstructure on $\mathbf {D}^b\big (\mathrm {MHM}(X)\big )$ , one obtains for every $u\in \mathbf { D}^b\big (\mathrm {MHM}(X)\big )$ and every $k\in {\mathbf Z}$ a spectral sequence
Given a morphism $f\colon Y\to X$ of smooth complex varieties, we use the notation $f_*$ for the pushforward of Hodge modules. Note that at the level of underlying $\mathscr {D}$ modules, this corresponds to the usual pushforward $f_+$ , defined for right $\mathscr {D}$ modules as
where the object on the right is in the derived category of right $\mathscr {D}_X$ modules. Here,
is the associated transfer $(\mathscr {D}_Y, f^{1} \mathscr {D}_X)$ bimodule, which is isomorphic to $f^* \mathscr {D}_X$ as an $\mathscr {O}_Y$ module, and is filtered by $f^* F_k \mathscr {D}_X$ (see [Reference Hotta, Takeuchi and Tanisaki18, §1.5] for more details). In general, $f_+$ is different from the pushforward $\mathbf {R} f_*$ on quasicoherent $\mathscr {O}$ modules, but the two definitions agree if f is an open immersion, and, in this case, we also use the $\mathbf {R} f_*$ notation.
Moreover, if f is proper and if we denote by $\mathrm {FM}(\mathscr {D}_X)$ the category of filtered $\mathscr {D}$ modules on X, then there is a functor
defined in [Reference Saito48], which is compatible with the functor $f_+$ above. If $(\mathcal {M},F)$ underlies a Hodge module M on X, we write $f_+(\mathcal {M},F)$ for the object in $\mathbf {D}^b\big (\mathrm {FM}(\mathscr {D}_Y)\big )$ underlying $f_* M$ .
An important feature of the pushforward of Hodge modules under projective morphisms is the strictness property of the Hodge filtration (see [Reference Saito49, Theorem 2.14]). This says that if $f\colon Y\to X$ is projective and $(\mathcal {M},F)$ underlies a Hodge module on Y, then $f_+ (\mathcal {M}, F)$ is strict as an object in $\mathbf {D}^b \big (\mathrm {FM}(\mathscr {D}_X)\big )$ (and moreover, each ${\mathcal H}^i f_+ (\mathcal {M}, F)$ underlies a Hodge module). Concretely, this means that the natural mapping
is injective for every $i, k\in {\mathbf Z}$ . The filtration on ${\mathcal H}^if_+(\mathcal {M},F)$ is obtained by taking $F_k{\mathcal H}^if_+(\mathcal {M},F)$ to be the image of this map. Note that this strictness property is a vast generalisation of the degeneration at $E_1$ of the Hodgetode Rham spectral sequence (cf., e.g. [Reference Mustaţă and Popa36, §4]).
2.2 Brief review of local cohomology
Let X be a smooth, irreducible ndimensional complex algebraic variety and Z a proper closed subscheme of X defined by the coherent ideal sheaf $\mathcal {I}_Z$ . For a quasicoherent $\mathscr {O}_X$ module $\mathcal {M}$ and $j\geq 0$ , we denote by ${\mathcal H}^q_Z(\mathcal {M})$ the $q^{\mathrm {th}}$ local cohomology sheaf of $\mathcal {M}$ , with support in Z. This is the $q^{\mathrm {th}}$ derived functor of the functor $\underline {\Gamma _Z} ()$ given by the subsheaf of local sections with support in Z. The sheaves ${\mathcal H}^q_Z(\mathcal {M})$ only depend on the support of Z, so in many situations, it is convenient to assume that Z is reduced. For the basic facts on local cohomology, see [Reference Hartshorne15].
The sheaf ${\mathcal H}^q_Z(\mathcal {M})$ is a quasicoherent sheaf, whose local sections are annihilated by suitable powers of $\mathcal {I}_Z$ . For every affine open subset $U\subseteq X$ , if
then ${\mathcal H}^q_Z(\mathcal {M})\vert _U$ is the sheaf associated to the local cohomology module $H^q_I(M)$ . This can be computed as follows: if $I=(f_1,\ldots ,f_r)$ (or, more generally, if I and $(f_1,\ldots ,f_r)$ have the same radical) and for a subset $J\subseteq \{1,\ldots ,r\}$ , we put $f_J:=\prod _{i\in J}f_i$ , then we have the Čechtype complex
with
and with the maps given (up to suitable signs) by the localisation homomorphisms. With this notation, we have functorial isomorphisms
The two main cases we will be interested in are those when $\mathcal {M}$ is either $\mathscr {O}_X$ or $\omega _X$ . Note that we have ${\mathcal H}^0_Z(\mathscr {O}_X) = {\mathcal H}^0_Z(\omega _X) = 0$ .
In general, given X and Z as above, we write $U=X\backslash Z$ and let $j\colon U\hookrightarrow X$ be the inclusion map. For every $\mathcal {M}$ , we have a functorial exact sequence
and functorial isomorphisms
Example 2.1. If Z has pure codimension 1 (that is, it is an effective divisor on X), then ${\mathcal H}^1_Z(\mathscr {O}_X)\simeq \mathscr {O}_X(*Z)/\mathscr {O}_X$ and ${\mathcal H}^q_Z(\mathscr {O}_X)=0$ for all $q\neq 1$ . Here, $\mathscr {O}_X(*Z)$ denotes the sheaf of rational functions on X with poles along Z.
Remark 2.2. Since X is smooth, hence, CohenMacaulay, the usual description of depth in terms of local cohomology (see [Reference Hartshorne15, Theorem 3.8]) implies that
Moreover, from the description via the Čech complex in (2.5), it follows that if Z is locally cut out settheoretically by $\leq N$ equations, then ${\mathcal H}^q_Z(\mathscr {O}_X)=0$ for $q>N$ . In particular, we conclude that if Z is a local complete intersection of pure codimension r, then ${\mathcal H}^q_Z(\mathscr {O}_X)=0$ for all $q\neq r$ .
If $Z'\subseteq Z$ is another closed subset of X, then for every quasicoherent sheaf $\mathcal {M}$ , we have natural maps
These are induced by the natural transformation of functors ${\mathcal H}^0_{Z'}()\to {\mathcal H}^0_Z()$ .
It is a standard fact that if $\mathcal {M}$ is a left $\mathscr {D}_X$ module, then each ${\mathcal H}_Z^q(\mathcal {M})$ has a canonical structure of left $\mathscr {D}_X$ module. Locally, this can be seen by using the description in (2.5). Indeed, each localisation $M_{f_J}$ has an induced $\mathscr {D}_X(U)$ module structure, such that the morphisms in the complex $C^{\bullet }\otimes _AM$ are morphisms of $\mathscr {D}_X(U)$ modules. Therefore, each cohomology module $H^q_I(M)$ has an induced $\mathscr {D}_X(U)$ module structure and one can easily check that this is independent of the choice of generators for I. Hence, these structures glue to a $\mathscr {D}_X$ module structure on ${\mathcal H}^q_Z(\mathcal {M})$ .
Note also that if $\mathcal {M}$ is a left $\mathscr {D}_X$ module, then the sheaves $R^qj_*(\mathcal {M}\vert _U)$ are left $\mathscr {D}_X$ modules as well; they are the cohomology sheaves of the $\mathscr {D}$ module pushforward of $\mathcal {M}$ via j. Moreover, in this case, the exact sequence (2.6) and the isomorphism (2.7) also hold at the level of $\mathscr {D}_X$ modules.
Similar considerations apply for right $\mathscr {D}_X$ modules. It is easy to see, using the local description, that for every $q\geq 0$ , we have a canonical isomorphism
In particular, we have a canonical isomorphism of right $\mathscr {D}_X$ modules
2.3 The Hodge filtration on local cohomology
We continue to use the notation introduced in the previous sections. Denoting by $i\colon Z\hookrightarrow X$ the inclusion, there are objects $j_*{\mathbf Q}_U^H[n]$ and $i_*i^!{\mathbf Q}_X^H[n]$ in the derived category of Hodge modules and an exact triangle
as shown in [Reference Saito49, § 4]. The cohomologies of these objects are Hodge modules whose underlying $\mathscr {D}_X$ modules we have already seen:
In particular, these $\mathscr {D}_X$ modules carry canonical Hodge filtrations; note that these only depend on the reduced subscheme $Z_{\mathrm {red}}$ . Moreover, the long exact sequence of cohomology corresponding to (2.8) gives the counterparts of (2.6) and (2.7) in this setting: an exact sequence of filtered $\mathscr {D}_X$ modules
and an isomorphism of filtered $\mathscr {D}_X$ modules
For example, when $Z = D$ is a reduced effective divisor, we have a canonical Hodge filtration on $\mathscr {O}_X(*D) = j_* \mathscr {O}_U$ , which underlies the Hodge module $j_*{\mathbf Q}_U^H[n]$ ; this is analysed in [Reference Mustaţă and Popa36]. Up to taking the quotient by $\mathscr {O}_X$ , this is therefore equivalent to the Hodge filtration on the local cohomology sheaf ${\mathcal H}^1_D(\mathscr {O}_X)$ . It turns out that for arbitrary Z, the Hodge filtration on the local cohomology sheaves ${\mathcal H}^q_Z(\mathscr {O}_X)$ can be defined using the one on sheaves of the form $\mathscr {O}_X(*D)$ . Note first that it is enough to describe the Hodge filtration in each affine chart U. In this case, if we consider $f_1,\ldots ,f_r\in A=\mathscr {O}_X(U)$ that generate an ideal having the same radical as $\mathcal {I}_Z(U)$ , then each localisation $A_{f_J}$ carries a Hodge filtration, such that the corresponding Čechtype complex (2.4) is a complex of filtered $\mathscr {D}$ modules. In fact, the maps come from morphisms of mixed Hodge modules: each component is, up to sign, induced by the canonical map
where $V\subseteq U$ are complements of suitable hypersurfaces and $j_U\colon U\hookrightarrow X$ and $j_V\colon V\hookrightarrow X$ are the inclusion maps. Therefore, the maps in the complex (2.4) are strict and the Hodge filtration on the cohomology sheaves ${\mathcal H}^q_Z(\mathscr {O}_X)$ is the induced filtration.
Remark 2.3. We have
Indeed, arguing locally and using the computation of local cohomology via the Čechtype complex (2.4), we deduce this assertion from the fact that if D is a reduced effective divisor on X, then $F_p\mathscr {O}_X(*D)=0$ for $p<0$ (see, for example, [Reference Mustaţă and Popa36, § 9]).
Remark 2.4. Suppose that $Z'\subseteq Z$ is another closed subset. If $U'=X\backslash Z'$ , then j factors as
In the derived category of Hodge modules on X, we have an isomorphism
The canonical morphism ${\mathbf Q}^H_{U'}[n]\to k_*{\mathbf Q}^H_U[n]$ induces therefore
so that after passing to cohomology, we get morphisms of Hodge modules
We deduce using (2.9) and (2.10) that the canonical morphisms
are (strict) morphisms of filtered $\mathscr {D}_X$ modules.
Remark 2.5. It is not hard to compare the Hodge filtrations on the local cohomology sheaves along Z with respect to two different embeddings in smooth varieties. Indeed, suppose that $k\colon X\hookrightarrow X'$ is a closed embedding, with $X'$ smooth, and $\dim (X)=n$ and $\dim (X')=n+d$ . In this case, if $i\colon Z\hookrightarrow X$ and $i'\colon Z\hookrightarrow X'$ are the two embeddings, we have an isomorphism
where we recall that $(d)$ denotes the Tate twist defined in §2.1. By taking cohomology, we see that for every q, we have an isomorphism of filtered $\mathscr {D}_{X'}$ modules
Explicitly, if $y_1,\ldots ,y_{n+d}$ are local coordinates on $X'$ , such that X is defined by $(y_1,\ldots ,y_d)$ , then we have an isomorphism
such that the Hodge filtrations are related by
Remark 2.6. For simplicity, we only considered the local cohomology of $\mathscr {O}_X$ . More generally, one can consider an arbitrary Hodge module M on X, with underlying filtered $\mathscr {D}_X$ module $\mathcal {M}$ . In this case, the local cohomology sheaves ${\mathcal H}_Z^q(\mathcal {M})$ continue to underlie Hodge modules and, thus, carry canonical Hodge filtrations.
We end this section with two examples: the case of a smooth subvariety and that of subsets defined by monomial ideals. We note, in addition, that the Hodge filtration on the local cohomology Hodge modules with support in generic determinantal ideals is studied extensively in [Reference Perlman and Raicu45] and [Reference Perlman44].
Example 2.7 (Smooth subvarieties)
If Z is a smooth, irreducible subvariety of X of codimension r, then ${\mathcal H}_Z^q(\mathscr {O}_X)=0$ for $q\neq r$ (see Remark 2.2). We claim that
In order to see this, we may assume that X is affine with $A=\mathscr {O}_X(X)$ and we have global algebraic coordinates $x_1,\ldots ,x_n$ on X, such that Z is defined by $I=(x_1,\ldots ,x_r)$ . In this case, as we have previously discussed, $H_I^r(A)$ is filtered isomorphic to the cokernel of the map
Moreover, since $x_1\cdots x_r$ defines a SNC divisor on X, the Hodge filtration on $A_{x_1\cdots x_r}$ is given by
(see [Reference Mustaţă and Popa36, § 8]). This is generated over A by the classes of $\frac {1}{x_1^{a_1}\cdots x_r^{a_r}}$ , with $a_1,\ldots ,a_r\geq 1$ , such that $a_1+\cdots +a_r=p+r$ . The fact that this is equal to the righthand side of (2.11) follows easily from the fact that $x_1,\ldots ,x_r$ form a regular sequence in A (for example, by reducing to the case when A is the polynomial ring ${\mathbf C}[x_1,\ldots ,x_r]$ ).
Another way to obtain this description, relying on the formalism of mixed Hodge modules, is the following. If $i\colon Z\hookrightarrow X$ is the inclusion, then
Applying $i_*$ and taking cohomology, we get an isomorphism of filtered $\mathscr {D}_X$ modules
Explicitly, if $x_1,\ldots ,x_n$ are local coordinates on X, such that Z is defined by $(x_1,\ldots ,x_r)$ , then we have an isomorphism
such that the Hodge filtration is given by
Note that the element $1\otimes 1$ corresponds to the class of $\frac {1}{x_1\cdots x_r}$ in the Čech description of local cohomology, hence, we obtain the same description of the Hodge filtration as in (2.11). For completeness, we note that it also follows from (2.12) that ${\mathcal H}^r_Z(\mathscr {O}_X)$ underlies a pure Hodge module of weight $n+r$ .
Example 2.8 (Monomial ideals)
We now consider the case when $X={\mathbf A}^n$ and $I\subseteq A={\mathbf C}[x_1,\ldots ,x_n]$ is a monomial ideal. The $({\mathbf C}^*)^n$ action on ${\mathbf A}^n$ induces a $({\mathbf C}^*)^n$ action on each $H^q_I(A)$ , which translates into a ${\mathbf Z}^n$ grading on $H^q_I(A)$ . The action of every element of $({\mathbf C}^*)^n$ induces an isomorphism of filtered $\mathscr {D}_X$ modules $H^q_I(A)\to H^q_I(A)$ , hence, every $F_pH^q_I(A)$ is a graded Asubmodule of $H^q_I(A)$ .
Let us denote by $e_1,\ldots ,e_n$ the standard basis of ${\mathbf Z}^n$ . It is easy to see that multiplication by $x_i$ induces an isomorphism
for every $u\in {\mathbf Z}^n$ , unless $u_i=1$ . This follows, for example, by computing the local cohomology via the Čechtype complex associated to a system of reduced monomial generators of $\mathrm {Rad}(I)$ (see [Reference Eisenbud, Mustaţǎ and Stillman10, Theorem 1.1]). Similarly, multiplication by $\partial _i$ induces an isomorphism
for every $u\in {\mathbf Z}^n$ , unless $u_i=0$ .
For every $u\in {\mathbf Z}^n$ , we write $x^u$ for the corresponding Laurent monomial. Given $J\subseteq \{1,\ldots ,n\}$ , let $u_J=\sum _{i\in J}e_i$ and $x_J=x^{u_J}$ . If for $u=(u_1,\ldots ,u_n)\in {\mathbf Z}^n$ , we put $\mathrm {neg}(u):=\{i\mid u_i<0\}$ , then $A_{x_J}=\bigoplus _{\mathrm {neg}(u)\subseteq J}{\mathbf C} x^u$ . Furthermore, the explicit description of the Hodge filtration on $A_{x_J}$ that we have seen in Example 2.7 can be rewritten as
where the direct sum is over those u with $\mathrm {neg}(u)\subseteq J$ and, such that, $\sum _{i\in \mathrm {neg}(u)}(u_i1)\leq p$ . Computing the local cohomology via the Čechtype complex, thus, gives
where the sum is over those $u\in {\mathbf Z}^n$ , such that $\sum _{i\in \mathrm {neg}(u)}(u_i1)\leq p$ . In particular, we see that
where the sum is over all $u\in {\mathbf Z}^n$ , such that $u_i\geq 1$ for all i.
2.4 Birational description and strictness
For us, it will be important to have a description of the Hodge filtration on ${\mathcal H}^q_Z(\mathscr {O}_X)$ via a log resolution of the pair $(X,Z)$ . To this end, it is more convenient to use the corresponding right Hodge modules, with corresponding $\mathscr {D}_X$ modules ${\mathcal H}^q_Z(\omega _X)$ .
Suppose that $f\colon Y\to X$ is such a resolution; more precisely, we require f to be a projective morphism that is an isomorphism over $U=X\backslash Z$ , such that Y is a smooth variety and the reduced inverse image $f^{1}(Z)_{\mathrm {red}}$ is a SNC divisor E. If $j'\colon V=Y\backslash E\hookrightarrow Y$ is the inclusion map, then we have a commutative diagram
in which the left vertical map is an isomorphism. We, thus, have an isomorphism
in the derived category of Hodge modules. As indicated in the paragraph after (2.10), since E is a divisor, the underlying filtered right $\mathscr {D}_Y$ module of the Hodge module $j^{\prime }_*{\mathbf Q}_V^H[n]$ is $\big (\omega _Y (*E), F\big )$ , where F is the Hodge filtration (see [Reference Mustaţă and Popa36, §8] for a more precise description in the present SNC setting). Taking cohomology in the isomorphism above, we, therefore, obtain isomorphisms of filtered right $\mathscr {D}_X$ modules
where for the last isomorphism, we use (2.9) and (2.10).
On the other hand, we have a filtered resolution of $\omega _Y(*E)$ by induced right $\mathscr {D}_Y$ modules, given by the complex
placed in degrees $n,\ldots ,0$ (see [Reference Mustaţă and Popa36, Proposition 3.1]). Using the notation and discussion in §2.1, by tensoring with $\mathscr {D}_{Y \to X}$ over $\mathscr {D}_Y$ , we, thus, obtain a filtered complex
which is filtered quasiisomorphic to $\omega _Y (*E) \overset {\mathbf {L}}{\otimes }_{\mathscr {D}_Y} \mathscr {D}_{Y\to X}$ and, therefore, can be used to compute $f_+ \omega _Y(*E)$ . Note that the filtration on $A^{\bullet }$ is, such that, $F_{pn}A^{\bullet }$ , for $p \ge 0$ , is the subcomplex
A detailed exposition of all this can be found in [Reference Mustaţă and Popa36, §2 and 3].
Now by (2.15) and the definition of $\mathscr {D}$ module pushforward, for all $q \ge 0$ , we have
as right $\mathscr {D}_X$ modules. The filtration on the righthand side is described at the end of §2.1, and since these filtered $\mathscr {D}$ modules underlie Hodge modules, the strictness property of the Hodge filtration in (2.3) leads to the following key Hodgetheoretic consequence for this birational interpretation:
Proposition 2.9. With the above notation, for every $p,q\geq 0$ , the inclusion $F_{pn}A^{\bullet }\hookrightarrow A^{\bullet }$ induces an injective map
whose image is $F_{pn}R^qj_*\omega _U$ . Moreover, via (2.15), this image coincides with $F_{pn} {\mathcal H}^{q+1}_Z(\omega _X)$ for $q \ge 1$ , while for $q=0$ , its quotient by $\mathscr {O}_X$ gives $F_{pn} {\mathcal H}^1_Z(\omega _X)$ .
As a concrete example, we obtain the following description of the lowest term of the filtration (note that by (2.1) and Remark 2.3, we have $F_p{\mathcal H}_Z^q(\omega _X)=0$ for all q and all $p<n$ ):
Corollary 2.10. For every $q\geq 2$ , we have a canonical isomorphism
Moreover, we have a short exact sequence
Proof. The assertion follows from Proposition 2.9 and the fact that $F_{n}A^{\bullet }=\omega _Y(E)$ .
Remark 2.11 (Analytic setting)
The results in this section also apply when Z is an analytic subspace of a complex manifold X, due to the study of open direct images in the analytic category (see [Reference Saito49, Proposition 2.11 and Corollary 2.20]) and the fact that the strictness theorem [Reference Saito49, Theorem 2.14] holds for any projective morphism between analytic spaces, for example, a resolution of singularities (cf. also [Reference Jung, Kim, Saito and Yoon23, §2.1]).
2.5 An injectivity theorem
We continue to use the notation in the previous section. Using the exact sequence
and the GrauertRiemenschneider vanishing theorem, Corollary 2.10 tells us that for all $q \ge 1$ , as a consequence of strictness for the Hodge filtration, we have an isomorphism
We also consider the inclusion maps
Theorem A in the Introduction follows, therefore, once the following compatibility is established:
Proposition 2.12. For each q, the composition
coincides with the morphism on cohomology described in the statement of Theorem A.
Proof. For simplicity, we write down the argument for $q \ge 2$ , when $ R^{q1} f_* \omega _E \simeq R^{q1} f_* \omega _Y(E)$ . The argument for $q =1$ is similar, only this time, by definition, one needs to consider $f_* \omega _Y(E) / \omega _Y$ (as opposed to $f_* \omega _Y(E)$ ).
Step 1. We consider the commutative diagram (2.14), and we identify U and V via the left vertical map. Applying $\mathbf {R} f_*$ to the canonical inclusion $\omega _Y (E) \hookrightarrow j^{\prime }_* \omega _U \simeq \omega _Y (*E)$ and passing to cohomology, we obtain morphisms
for each q, where the last isomorphism is the canonical isomorphism in (2.7). We claim that these morphisms can be identified with the compositions $i_q \circ \gamma _q$ .
To see this, recall that we have identified
The transfer module admits a canonical morphism $\mathscr {D}_Y \to \mathscr {D}_{Y \to X}$ of left $\mathscr {D}_Y$ modules (induced by $T_Y \to f^* T_X$ ), which, in turn, induces a morphism
Now the morphism $i_q \circ \gamma _q$ , defined using the resolution (2.16) of $\omega _Y(*E)$ , is obtained more precisely by pushing forward the inclusion $\omega _Y (E) \hookrightarrow \omega _Y (*E)$ , and then considering the composition
But since $f \circ j' = j$ , $\rho _q$ can also be identified canonically with the isomorphism $R^{q1} j_* \omega _U \to {\mathcal H}^{q1} j_+ \omega _U$ appearing above, and we are done.
Step 2. In this step, we discuss the following general situation: assume that W is a closed subscheme in X, with ideal sheaf ${\mathcal J}$ . If $j \colon V \hookrightarrow X$ is the inclusion map of the complement $V = X \backslash W$ , we have a diagram of exact triangles
where the vertical map on the right is the canonical morphism, and where the middle vertical map can be described as the canonical morphism
since ${\mathcal J}_{V} \simeq \mathscr {O}_V$ .
Moreover, in the presence of a proper morphism $f \colon Y \to X$ , assumed to be an isomorphism over V, we can consider the ideal ${\mathcal J}_Y : = {\mathcal J} \cdot \mathscr {O}_Y$ defining $f^{1} (W)$ , and, similarly, we have a canonical morphism
where $j'$ is the inclusion of V in Y. Applying $\mathbf {R} f_*$ to this morphism, and Grothendieck duality to the first term, we obtain a diagram
where we included the natural factorisation of the bottom morphism through the object $\mathbf {R} \mathcal {H} om_{\mathscr {O}_X} ({\mathcal J} , \omega _X)$ , induced by the canonical morphism ${\mathcal J} \to \mathbf {R} f_* {\mathcal J}_Y$ ; this factorisation holds due to the description of the morphism in (2.18) and the fact that Grothendieck duality is compatible with restriction to open subsets.
Step 3. In this step, we apply the constructions in Step 2 to give another description of the morphism (2.17), which will finish the proof. First, (2.17) can be rewritten as the composition
where the factorisation through the middle term holds since E is the reduced structure on $f^{1} (Z)$ . Applying Grothendieck duality, this composition can be rewritten as
and the map on the right factors further through $\mathscr {E} xt^{q1} \big ( \mathcal {I}_Z, \omega _X \big )$ , as described in the last diagram in Step 2. Moreover, it is straightforward to see that for $q \ge 2$ , we have canonical isomorphisms
and
Altogether, the morphism (2.17) can be identified with the natural composition
which is the same as the map on cohomology described in the statement of Theorem A (note that Grothendieck duality gives $ \mathbf {R} f_* \omega _E^{\bullet } \simeq \mathbf {R} \mathcal {H} om_{\mathscr {O}_X} (\mathbf {R} f_* \mathscr {O}_E, \omega _X[n])$ , while $\omega _Z^{\bullet } \simeq \mathbf {R} \mathcal {H} om_{\mathscr {O}_X} (\mathscr {O}_Z, \omega _X[n])$ ).
The upshot of Theorem A (and Proposition 2.12) is that for each $q\ge 1$ , we have a commutative diagram
where the left vertical map is the isomorphism in Corollary 2.10, the bottom horizontal map is the inclusion, the right vertical map is the natural map to local cohomology, while $\alpha _q$ is the morphism on cohomology induced by
in $\mathbf {D}^b\big (\mathrm {Coh} (X)\big )$ , obtained, in turn, by dualising the natural morphism $\mathscr {O}_Z \to \mathbf {R} f_* \mathscr {O}_E$ (here, $\omega _E^{\bullet } = \omega _E [n 1]$ since E is Gorenstein).
As a consequence, the top horizontal map is injective. In other words, this gives another proof of the injectivity result of Kovács and Schwede, Corollary B in the Introduction, applied in [Reference Kovács and Schwede29] to the study of deformations of Du Bois singularities.
2.6 A local vanishing theorem
We use the constructions in §2.4 to prove a Nakanotype vanishing result for log resolutions of arbitrary closed subsets. This generalises a result for hypersurfaces due to Saito [Reference Saito52, Corollary 3] (cf. also [Reference Mustaţă and Popa36, Theorem 32.1]).
Proof of Theorem D
We note that by the definition of c and by Remark 2.2, we have
The vanishing in the statement holds trivially for $p>n$ , hence, we may assume $p\leq n$ . Note that, in this case, the conditions in both 1 and 2 imply $q\geq 1$ . We first check the case $p =n$ .Footnote ^{2} This follows in both cases 1 and 2 from (2.19) and Corollary 2.10.
We next prove the theorem by descending induction on p. Let $p < n$ , and consider the complex
placed in cohomological degrees $0,\ldots ,np$ , where $A^{\bullet }$ is as in § 2.4. Note that since $p+q\geq n+1$ , we deduce from Proposition 2.9 that
hence, using again (2.19), we see that in both cases 1 and 2, we have
Consider now the hypercohomology spectral sequence
By definition, we have
hence, we want to show that $E_1^{0,q}=0$ . First, (2.20) implies that in both cases 1 and 2, we have $E_{\infty }^{0,q}=0$ .
Now for every $k \ge 1$ , we clearly have $E_k^{k,q+k1}=0$ , since this is a firstquadrant spectral sequence. On the other hand, we also have $E_k^{k,qk+1}=0$ by induction. Indeed, this is a subquotient of
and the righthand side vanishes by induction. We, thus, conclude that $E_1^{0,q}=E_{\infty }^{0,q}=0$ , completing the proof.
Remark 2.13. (1) In the statement of the theorem, we may replace $c = \mathrm {lcd}(Z, X)$ by any s, such that Z is locally cut out by s equations. Indeed, the fact that $c\leq s$ follows from Remark 2.2.
(2) There exist further useful upper bounds on c that depend only on the codimension r of Z. We only list a couple here. In complete generality, Faltings [Reference Faltings13] showed that
Among many other improvements, Huneke and Lyubeznik [Reference Huneke and Lyubeznik21] showed that if Z is normal, then
Further results along these lines, assuming $S_k$ conditions on Z, appear in [Reference Dao and Takagi8]. We obtain, in particular:
Corollary 2.14. Let Z be a closed subscheme of codimension r in a smooth, irreducible ndimensional variety X. If $f\colon Y \to X$ is a log resolution of $(X, Z)$ , which is an isomorphism away from Z, and $E = f^{1}(Z)_{\mathrm {red}}$ , then
If, moreover, Z is assumed to be normal, then the same holds for
We conclude by noting that in [Reference Mustaţă and Popa36, Theorem 32.1], it is shown that when Z is a Cartier divisor, in order to have local vanishing as in Theorem D, it is enough to assume only that X is smooth away from Z. It is, therefore, natural to ask:
Question 2.21. Is there an appropriate generalisation of Theorem D that does not assume X to be smooth?
3 Order and Ext filtrations, and some comparisons
3.1 Order and Ext filtration
We now aim to define analogues of the pole order filtration associated to hypersurfaces and compare them with the Hodge filtration. We thank C. Raicu, whose answers to our questions have helped shape the material in this section. We start by recording the following basic property of the Hodge filtration:
Proposition 3.1. For every $p,q\geq 0$ , we have
In particular, we have
Proof. The first assertion is a general property of filtered $\mathscr {D}_X$ modules underlying Hodge modules whose support is contained in Z (see [Reference Saito48, Lemma 3.2.6]). The second assertion then follows from the fact that $F_{1}{\mathcal H}^q_Z(\mathscr {O}_X)=0$ (see Remark 2.3).
Remark 3.2. If Z is a reduced divisor, the second assertion in the above proposition is equivalent with the fact that $F_p\mathscr {O}_X(*Z)\subseteq \mathscr {O}_X\big ((p+1)Z\big )$ , that is, the Hodge filtration is contained in the pole order filtration (see [Reference Saito50, Proposition 0.9] and also [Reference Mustaţă and Popa36, Lemma 9.2]). Moreover, in terms of Hodge ideals, the first assertion says that $I_p(Z)\subseteq I_{p1}(Z)$ (see [Reference Mustaţă and Popa36, Proposition 13.1]).
Definition 3.3. The order filtration on ${\mathcal H}^q_Z(\mathscr {O}_X)$ is the increasing filtration given by
Note that we have a canonical isomorphism
If Z is a reduced divisor, then
and the inclusions $F_k \mathscr {O}_X(*Z) \subseteq \mathscr {O}_X \big ((k+1)Z\big )$ in Remark 3.2 say that
This last fact continues to be true, in general:
Proposition 3.4. For arbitrary Z, and for every k and q, we have
Proof. The statement is equivalent to the second assertion in Proposition 3.1.
Remark 3.5. The order filtration on ${\mathcal H}^q_Z(\mathscr {O}_X)$ is compatible with the filtration on $\mathscr {D}_X$ by order of differential operators:
However, unless $q = r = \mathrm {codim}_X(Z)$ , the sheaves $O_k{\mathcal H}^q_Z(\mathscr {O}_X)$ are not coherent as long as ${\mathcal H}^q_Z(\mathscr {O}_X)\neq 0$ . This makes the order filtration less suitable for $q> r$ ; this is a rather deep result of Lyubeznik [Reference Lyubeznik32, Corollary 3.5].Footnote ^{3} For $q = r$ , the situation is better (see Proposition 3.11 below).
We also consider a related filtration. Recall that a wellknown characterisation of local cohomology (see [Reference Hartshorne15, Theorem 2.8]) is
where the morphisms
between the terms in the direct limit are induced from the short exact sequence
Definition 3.6. The Ext filtration on ${\mathcal H}^q_Z(\mathscr {O}_X)$ is the increasing filtration given by
Remark 3.7. It is clear from the definitions of the order and Ext filtrations that they depend on the schemetheoretic structure of Z and not just on the underlying set. A natural choice, which gives the deepest such filtrations, is to take Z to be reduced. However, it can be convenient to have the flexibility of allowing filtrations associated to nonreduced schemes (for example, in the case of settheoretic complete intersections).
Remark 3.8. If $q = r = \mathrm {codim}_X(Z)$ , then
that is, the maps in the above definition are injective. Indeed, in this case, the maps in (3.1) are all injective, since
This last fact follows from the following well known (see, e.g. [Reference Bănică and Stănăşilă3, Proposition 1.17]):
Lemma 3.9. If $\mathscr {F}$ is a coherent sheaf on a smooth variety, then
In terms of comparing these two natural filtrations, we clearly have
Remark 3.10. This time, we obviously have that $E_k{\mathcal H}^q_Z(\mathscr {O}_X)$ are coherent sheaves; by Remark 3.5, this means, in particular, that $E_k \neq O_k$ when $q>r$ and ${\mathcal H}^q_Z(\mathscr {O}_X)\neq 0$ . On the other hand, in general, it is not clear any more whether the Ext filtration is compatible with the filtration on $\mathscr {D}_X$ .
Let’s try to understand the inclusion (3.2) more canonically. According to the proof of [Reference Huneke and Koh20, Proposition 3.1(i)], for any ideal sheaf $\mathcal {J}\subseteq \mathscr {O}_X$ , such that $\mathrm {Supp} \left (\mathscr {O}_X/ \mathcal {J} \right ) = Z$ , there is a spectral sequence
which is simply the spectral sequence of the composition of the functors ${\mathcal H}^0_Z ()$ and ${\mathcal H} om_{\mathscr {O}_X} (\mathscr {O}_X/\mathcal {J}, \cdot )$ , since
for every $\mathscr {O}_X$ module $\mathcal {M}$ , and ${\mathcal H}^0_Z ()$ takes injective objects to injective objects.
Hence, taking $\mathcal {J} = \mathcal {I}_Z^{k+1}$ , in general, the picture is this: $O_k{\mathcal H}^q_Z(\mathscr {O}_X)$ is the $E^{0,q}_2$ term of this spectral sequence, we have $E^{0,q}_\infty \hookrightarrow E^{0,q}_2$ (as there are no nontrivial differentials coming into $E^{0,q}_r$ ), while $E^{0,q}_\infty $ is a quotient of $H^q = \mathscr {E} xt^{q}_{\mathscr {O}_X} \big (\mathscr {O}_X/ \mathcal {I}_Z^{k+1}, \mathscr {O}_X\big )$ , which is identified with $E_k{\mathcal H}^q_Z(\mathscr {O}_X)$ .
The drawbacks for both the order and the Ext filtration disappear when $q =r=\mathrm {codim}_X(Z)$ due to the following:
Proposition 3.11. We have $E_k{\mathcal H}^r_Z(\mathscr {O}_X) = O_k{\mathcal H}^r_Z(\mathscr {O}_X)$ for all $k \ge 0$ .
Proof. Using the spectral sequence (3.3), since ${\mathcal H}^q_Z(\mathscr {O}_X) = 0$ for $q < r$ , for any ideal sheaf $\mathcal {J}\subseteq \mathscr {O}_X$ , such that $\mathrm {Supp} \left (\mathscr {O}_X/ \mathcal {J} \right ) = Z$ , we have
The statement follows again by taking $\mathcal {J} = \mathcal {I}_Z^{k+1}$ (see also Remark 3.8).
In particular, for $q =r$ , Proposition 3.4 can be reinterpreted as saying that
A natural question, potentially interesting for the study of the singularities of Z, is whether this extends to higher values of q as well.
Question 3.5. (When) do we have inclusions $F_k {\mathcal H}_Z^q(\mathscr {O}_X) \subseteq E_k {\mathcal H}_Z^q(\mathscr {O}_X)$ for $q> r$ ?
We will answer this question positively for $k =0$ and all q in Proposition 3.15 below. However, we first note that in the smooth case, we, indeed, have equality between all three filtrations, as expected. If Z is a smooth, irreducible subvariety of X of codimension r, then ${\mathcal H}_Z^q(\mathscr {O}_X)=0$ for $q\neq r$ (just as for any local complete intersection; see Remark 2.2).
Example 3.12. If Z is a smooth, irreducible subvariety of X of codimension r, then
Indeed, we have seen the first equality in Example 2.7 and the second equality follows from Proposition 3.11.
Remark 3.13. We will see in Corollary 3.26 below that if Z is a singular local complete intersection in X, of pure codimension r, then $F_k{\mathcal H}^r_Z(\mathscr {O}_X) \neq O_k {\mathcal H}_Z^r(\mathscr {O}_X)$ for $k\gg 0$ . However, this can fail beyond the local complete intersection case, even for nice varieties. For example, C. Raicu pointed out to us that if X is the variety of $m\times n$ matrices, with $m>n$ , and Z is the subset consisting of matrices of rank $\leq p$ (so that $\mathrm {codim}_X(Z)=r=(mp)(np)$ , then one can show using [Reference Perlman44, Corollary 1.6] that
Remark 3.14. Another way of thinking about the isomorphism
when Z is smooth is in terms of the description in Corollary 2.10. Indeed, considering the log resolution of $(X, Z)$ to be the blow up of X along Z, it translates into the isomorphism $R^{r1}f_* \omega _E \simeq \omega _Z$ , which is well known.
As promised, in general, we have a positive answer to Question 3.5 for the lowest piece of the Hodge filtration.
Proposition 3.15. For every $q \ge 0$ , we have an inclusion $F_0 {\mathcal H}^q_Z(\mathscr {O}_X) \subseteq E_0{\mathcal H}^q_Z(\mathscr {O}_X)$ .
Proof. Equivalently, the statement says that there is an inclusion
This is an immediate consequence of Theorem A, in which we established the existence of commutative diagrams
where the diagonal map is injective, identified with the inclusion $F_{n}{\mathcal H}^q_Z(\omega _X) \hookrightarrow {\mathcal H}^q_Z(\omega _X)$ via Corollary 2.10.
Remark 3.16 (Normal schemes)
If Z is normal of codimension r, then the inclusion
has an alternative interpretation: since Z is normal, the dualising sheaf
and the canonical sheaf $i_* \omega _U$ , where $i\colon U\hookrightarrow Z$ is the inclusion of the smooth locus of Z, are isomorphic. Since $\omega _Z \otimes \omega _X^{1}$ is reflexive, and coincides with $F_0 {\mathcal H}_Z^r(\mathscr {O}_X)$ on U, the conclusion follows.
3.2 The lowest term and Du Bois singularities
When Z is a reduced divisor, it is known that the pair $(X,Z)$ is log canonical if and only if Z has du Bois singularities (see [Reference Kovács and Schwede28, Corollary 6.6]). On the other hand, the condition of being log canonical is equivalent to the equality $F_0 \mathscr {O}_X(*Z) = P_0 \mathscr {O}_X(*Z)$ , where $P_\bullet $ is the pole order filtration or, in other words, $I_0 (Z) = \mathscr {O}_X$ in the language of Hodge ideals (see [Reference Mustaţă and Popa36, Corollary 10.3]).
We now show that Theorem A (and the discussion in §2.5), together with a criterion for Du Bois singularities due to Steenbrink and Schwede, imply that, in general, if Z has Du Bois singularities, then $F_0 {\mathcal H}_Z^q(\mathscr {O}_X) = E_0 {\mathcal H}_Z^q(\mathscr {O}_X)$ for all q; moreover, the converse holds if Z is CohenMacaulay.
Proof of Theorem C
It follows from work of Steenbrink [Reference Steenbrink59] (see Theorem 5.1 below) that Z has Du Bois singularities if and only if the canonical morphism $\mathscr {O}_Z\to \mathbf {R} f_*\mathscr {O}_E$ is an isomorphism (see also Schwede’s [Reference Schwede55, Theorem 4.6] for a more general criterion). Via duality, this is equivalent to the map
in the statement of Theorem A being an isomorphism, hence, to the horizontal map in (3.6) being an isomorphism for each q. This shows the first assertion.
Under the extra assumption of Z being CohenMacaulay of pure codimension r, we have
so by Proposition 3.15, we also have $F_0{\mathcal H}^q_Z(\mathscr {O}_X) = 0$ for all $q \neq r$ . Now as explained in Remark 3.8, for $q =r$ , the vertical map in (3.6) is injective, so $F_{n}{\mathcal H}^r_Z(\omega _X)= E_{n}{\mathcal H}^r_Z(\omega _X)$ is equivalent (cf. Proposition 2.12) to the canonical map
being an isomorphism. Since all the other $\mathscr {E} xt$ sheaves are zero, it follows (using Grothendieck duality) that the morphism $\alpha $ is an isomorphism, which, as noted, is equivalent to Z being Du Bois.
Remark 3.17 (NonCohenMacaulay case)
L. Ma has pointed out that when Z is not CohenMacaulay, it can happen that Z is not Du Bois, but $F_0 {\mathcal H}_Z^q(\mathscr {O}_X) = E_0 {\mathcal H}_Z^q(\mathscr {O}_X)$ for all q. For example, this is the case if $Z=\mathrm {Spec}\big ({\mathbf C}[s^4,s^3t,st^3,t^4]\big )\hookrightarrow {\mathbf A}^4$ . We leave the argument for Chapter 5, in which we discuss some basic facts about Du Bois complexes (see Example 5.4).
On a related note, the following corollary of Theorem A recovers [Reference Ma, Schwede and Shimomoto33, Theorem B] in the case when the ambient space X is smooth (see also Remark 3.19 below for the general case of this result).
Corollary 3.18. If $Z\subseteq X$ is a closed subscheme with Du Bois singularities, then the natural maps
are injective for all q.
Proof. As above, if Z is Du Bois, the natural morphism $\mathscr {O}_Z \to \mathbf {R} f_* \mathscr {O}_E$ is an isomorphism, and, therefore, the canonical maps
are isomorphisms for each q. The result then follows from Theorem A.
Remark 3.19. The question of when this injectivity holds is asked in [Reference Eisenbud, Mustaţǎ and Stillman10] (see [Reference Ma, Schwede and Shimomoto33] for further discussion and applications). In fact, as L. Ma has pointed out, one can deduce from Corollary 3.18 the full statement of [Reference Ma, Schwede and Shimomoto33, Theorem B] (in which the ambient variety X is only assumed to be Gorenstein). Indeed, we may assume that $X=\mathrm {Spec}(R)$ and $Z=\mathrm {Spec}(S)$ are affine, with $S=R/I$ . The assertion in the corollary implies via [Reference Dao, De Stefani and Ma7, Proposition 2.1] that S is icohomologically full for every i (equivalently, in the language of [Reference Kollár and Kovács26], S has liftable local cohomology). This implies that for every maximal ideal ${\mathfrak m}$ of R and every i and k, the natural map $H_{{\mathfrak m}}^i(R/I^k)\to H^i_{{\mathfrak m}}(R/I)$ is surjective. If R is Gorenstein, then $\omega _{R_{{\mathfrak m}}}\simeq R_{\mathfrak m}$ , and local duality implies that the natural map
is injective, where $n=\mathrm {dim}(R_{\mathfrak m})$ . By taking the direct limit over k, we obtain the injectivity of
We conclude this section by noting that a study of the equality $F_1 {\mathcal H}_Z^q(\mathscr {O}_X) = E_1 {\mathcal H}_Z^q(\mathscr {O}_X)$ should also be very interesting. Recall that in [Reference Mustaţă and Popa36, Theorem C], it is shown that for a reduced hypersurface D, the equality $F_1 \mathscr {O}_X(*D) = P_1 \mathscr {O}_X(*D)$ (or, equivalently, $I_1 (D) = \mathscr {O}_X$ ) implies that D has rational singularities. By analogy we make the following:
Conjecture 3.20. If Z is a local complete intersection of pure codimension r in X and if $F_1 {\mathcal H}_Z^r(\mathscr {O}_X) = E_1{\mathcal H}_Z^r(\mathscr {O}_X)$ , then Z has rational singularities.
We will show in Lemma 3.23 below that in the setting of the conjecture, the condition $F_1=E_1$ implies that $F_0=E_0$ also holds. We also note that the assertion in the conjecture follows from the stronger Conjecture 3.31 below. It is an interesting question whether a similar condition on the Hodge filtration on all local cohomology sheaves ${\mathcal H}^q_Z(\mathscr {O}_X)$ would imply the fact that Z has rational singularities for any reduced Z.
3.3 The case of local complete intersections
All throughout this section, Z is assumed to be a local complete intersection subscheme of X, of pure codimension r, defined by the ideal $\mathcal {I}_Z$ . In this case, we have ${\mathcal H}^q_Z(\mathscr {O}_X)=0$ for $q\neq r$ by Remark 2.2. We have seen in Propositions 3.4 and 3.11 that
for all $k \ge 0$ .
We start by giving more precise descriptions of the order and Ext filtrations; even though they coincide, each of the two filtration provides interesting information. We denote by $\mathscr {N}_{Z/X}$ the normal sheaf $(\mathcal {I}_Z/\mathcal {I}_Z^2)^{\vee }$ .
Lemma 3.21. For every $k\geq 0$ , the quotient
is a locally free $\mathscr {O}_Z$ module; in fact, it is isomorphic to $\mathrm {Sym}^{k}(\mathscr {N}_{Z/X})\otimes \omega _Z\otimes \omega _X^{1}$ .
Proof. By Remark 3.8, we have $E_k{\mathcal H}^r_Z(\mathscr {O}_X) = {\mathcal Ext}_{\mathscr {O}_X}^r(\mathscr {O}_X/\mathcal {I}_Z^{k+1},\mathscr {O}_X)$ , and, moreover, each exact sequence
induces an exact sequence
We, thus, see that
Furthermore, working locally, we may assume that Z is the closed subscheme of X defined by $f_1,\ldots ,f_r\in \mathscr {O}_X(X)$ , with $f_1,\ldots ,f_r$ forming a regular sequence at every point of Z. Given this, an easy computation shows that as in the case of smooth subvarieties in Example 2.7, we have:
Lemma 3.22. The sheaf $O_k{\mathcal H}^r_Z(\mathscr {O}_X)$ is generated over $\mathscr {O}_X$ by the classes of $\frac {1}{f_1^{a_1}\cdots f_r^{a_r}}$ , where $a_1,\ldots ,a_r\geq 1$ , with $\sum _ia_i\leq k+r$ .
By analogy with the case of hypersurfaces [Reference Mustaţă and Popa36], one of the main questions to understand is when, given $p\geq 0$ , we have
We first note that it suffices to check the equality for $k = p$ :
Lemma 3.23. If $F_p {\mathcal H}_Z^r(\mathscr {O}_X)= O_p {\mathcal H}^r_Z(\mathscr {O}_X)$ , then
Proof. It suffices to check this for $k = p1$ . We use the notation $F_k$ and $O_k$ for simplicity. Note first that by Propositions 3.1 and 3.4, we have
On the other hand, a brief inspection of the concrete description of $O_k$ given in Lemma 3.22 shows that $\mathcal {I}_Z \cdot O_p = O_{p1}$ . We conclude by combining these two facts.
The next lemma shows that this question regarding the comparison between the Hodge and order filtration is interesting only if we assume that Z is reduced.
Lemma 3.24. If Z is nonreduced, then $F_0{\mathcal H}_Z^r(\mathscr {O}_X)\neq O_0{\mathcal H}^r_Z(\mathscr {O}_X)$ .
Proof. Let $O^{\prime }_k{\mathcal H}^r_Z(\mathscr {O}_X)$ be the order filtration on ${\mathcal H}^r_Z(\mathscr {O}_X)$ corresponding to $Z_{\mathrm {red}}$ . Since we have the inclusions
it is enough to show that $O^{\prime }_0{\mathcal H}^r_Z(\mathscr {O}_X)\neq O_0{\mathcal H}^r_Z(\mathscr {O}_X)$ .
Note that Z is CohenMacaulay, being a local complete intersection; since it is not reduced, it is not generically reduced. After restricting to a suitable open subset, we may, thus, assume that X is affine, with coordinates $x_1,\ldots ,x_n$ , such that the ideal of $Z_{\mathrm {red}}$ is generated by $x_1,\ldots ,x_r$ , and if we denote by $f_1,\ldots ,f_r$ the generators of $\mathcal {I}_Z$ , and write $f_i=\sum _{j=1}^ra_{i,j}x_j$ , then $\mathrm {det}(a_{i,j})\in (x_1,\ldots ,x_r)$ . The assertion in the lemma follows from the fact that via the isomorphisms
given by the Čechcomplex description in §2.2, the class of $\frac {1}{x_1\cdots x_r}$ , which generates $O^{\prime }_0{\mathcal H}_Z^r(\mathscr {O}_X)$ , corresponds to $\mathrm {det}(a_{i,j})\frac {1}{f_1\cdots f_r}$ . On the other hand, $O_0{\mathcal H}^r_Z(\mathscr {O}_X)$ is generated by the class of $\frac {1}{f_1\cdots f_r}$ , hence, it is different from $O^{\prime }_0{\mathcal H}_Z^r(\mathscr {O}_X)$ .
Before stating the next result, we note that a stronger bound will be obtained in Theorem 3.39, however, with much more work; the simple argument here is sufficient for establishing Corollary 3.26.
Proposition 3.25. If Z is not smooth, then
Proof. We may assume that X is affine and $\mathcal {I}_Z$ is generated by $f_1,\ldots ,f_r$ . Since Z is not smooth, it follows that there is a point $Q\in Z$ defined by the ideal $\mathcal {I}_Q$ , such that, after possibly renumbering and replacing $f_1$ by a linear combination of $f_1,\ldots ,f_r$ , we have $f_1\in \mathcal {I}_Q^2$ . We now need to appeal to a result that will be proved later, Theorem 4.2, saying that the Hodge filtration on ${\mathcal H}_Z^r(\mathscr {O}_X)$ is generated at level $nr$ . If $k\geq nr+1$ , we, thus, have
Recall that $O_{k1}{\mathcal H}_Z^r(\mathscr {O}_X)$ is generated by the classes of $\frac {1}{f_1^{a_1}\cdots f_r^{a_r}}$ , with $a_i\geq 1$ for all i and $\sum _ia_i\leq k1+r$ . Moreover, using the fact that $f_1,\ldots ,f_r$ form a regular sequence in $\mathscr {O}_{X,Q}$ , it is easy to see that these elements form a minimal system of generators of $O_{k1}{\mathcal H}_Z^r(\mathscr {O}_X)$ at Q. A similar assertion holds for $O_k{\mathcal H}_Z^r(\mathscr {O}_X)$ .
Since $f_1\in \mathcal {I}_Q^2$ , a straightforward calculation shows that
where the last sum is over those $a_1,\ldots ,a_r$ , such that $a_i\geq 1$ for all i, with the inequality being strict for some $i\geq 2$ , and, such that, $\sum _ia_i=k+r$ . This shows that $F_k{\mathcal H}_Z^r(\mathscr {O}_X)$ is a proper subset of $O_k{\mathcal H}_Z^r(\mathscr {O}_X)$ at Q.
In particular, the coincidence of the two filtrations characterises smoothness, similar to [Reference Mustaţă and Popa36, Theorem A] for hypersurfaces (for another approach to the same result, see Theorem 3.39 below).
Corollary 3.26. Z is smooth if and only if $F_k{\mathcal H}_Z^r(\mathscr {O}_X) = O_k{\mathcal H}^r_Z(\mathscr {O}_X)$ for all k.
We formalise the discussion above by introducing a measure of singularities that will figure prominently in the study of the Du Bois complex of Z.
Definition 3.27. The singularity level of the Hodge filtration on $\mathcal {H}^r_Z \mathscr {O}_X$ is
with the convention that $p (Z) = 1$ if equality never holds.
Remark 3.28. The invariant $p(Z)$ only depends on Z and not on the embedding in a smooth, ambient variety. In order to see this, let us temporarily denote by $p(Z\hookrightarrow X)$ the invariant corresponding to a closed embedding in a smooth variety X. Given a closed immersion of smooth varieties $X\hookrightarrow Y$ , by Remark 2.5, we have
Suppose next that Z is affine, and consider closed immersions $i\colon Z\hookrightarrow X$ and $i'\colon Z\hookrightarrow X'$ , with X and $X'$ smooth and affine. In order to show that $p(Z\hookrightarrow X)=p(Z\hookrightarrow X')$ , by the identity above, we may assume that