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We study the existence and structure of branch points in two-phase free boundary problems. More precisely, we construct a family of minimizers to an Alt–Caffarelli–Friedman-type functional whose free boundaries contain branch points in the strict interior of the domain. We also give an example showing that branch points in the free boundary of almost-minimizers of the same functional can have very little structure. This last example stands in contrast with recent results of De Philippis, Spolaor and Velichkov on the structure of branch points in the free boundary of stationary solutions.
Let G be a finite group. Let
$H, K$
be subgroups of G and
$H \backslash G / K$
the double coset space. If Q is a probability on G which is constant on conjugacy classes (
$Q(s^{-1} t s) = Q(t)$
), then the random walk driven by Q on G projects to a Markov chain on
$H \backslash G /K$
. This allows analysis of the lumped chain using the representation theory of G. Examples include coagulation-fragmentation processes and natural Markov chains on contingency tables. Our main example projects the random transvections walk on
$GL_n(q)$
onto a Markov chain on
$S_n$
via the Bruhat decomposition. The chain on
$S_n$
has a Mallows stationary distribution and interesting mixing time behavior. The projection illuminates the combinatorics of Gaussian elimination. Along the way, we give a representation of the sum of transvections in the Hecke algebra of double cosets, which describes the Markov chain as a mixture of Metropolis chains. Some extensions and examples of double coset Markov chains with G a compact group are discussed.
The space of Fredholm operators of fixed index is stratified by submanifolds according to the dimension of the kernel. Geometric considerations often lead to questions about the intersections of concrete families of elliptic operators with these submanifolds: Are the intersections nonempty? Are they smooth? What are their codimensions? The purpose of this article is to develop tools to address these questions in equivariant situations. An important motivation for this work are transversality questions for multiple covers of J-holomorphic maps. As an application, we use our framework to give a concise exposition of Wendl’s proof of the superrigidity conjecture.
We introduce a conjecture on Virasoro constraints for the moduli space of stable sheaves on a smooth projective surface. These generalise the Virasoro constraints on the Hilbert scheme of a surface found by Moreira and Moreira, Oblomkov, Okounkov and Pandharipande. We verify the conjecture in many nontrivial cases by using a combinatorial description of equivariant sheaves found by Klyachko.
Given a profinite group G of finite p-cohomological dimension and a pro-p quotient H of G by a closed normal subgroup N, we study the filtration on the Iwasawa cohomology of N by powers of the augmentation ideal in the group algebra of H. We show that the graded pieces are related to the cohomology of G via analogues of Bockstein maps for the powers of the augmentation ideal. For certain groups H, we relate the values of these generalized Bockstein maps to Massey products relative to a restricted class of defining systems depending on H. We apply our study to prove lower bounds on the p-ranks of class groups of certain nonabelian extensions of
$\mathbb {Q}$
and to give a new proof of the vanishing of Massey triple products in Galois cohomology.
Jannsen asked whether the rational cycle class map in continuous
$\ell $
-adic cohomology is injective, in every codimension for all smooth projective varieties over a field of finite type over the prime field. As recently pointed out by Schreieder, the integral version of Jannsen’s question is also of interest. We exhibit several examples showing that the answer to the integral version is negative in general. Our examples also have consequences for the coniveau filtration on Chow groups and the transcendental Abel-Jacobi map constructed by Schreieder.
Let V be a smooth quasi-projective complex surface such that the first three logarithmic plurigenera
$\overline P_1(V)$
,
$\overline P_2(V)$
and
$\overline P_3(V)$
are equal to 1 and the logarithmic irregularity
$\overline q(V)$
is equal to
$2$
. We prove that the quasi-Albanese morphism
$a_V\colon V\to A(V)$
is birational and there exists a finite set S such that
$a_V$
is proper over
$A(V)\setminus S$
, thus giving a sharp effective version of a classical result of Iitaka [12].
Let F be a CM number field. We generalise existing automorphy lifting theorems for regular residually irreducible p-adic Galois representations over F by relaxing the big image assumption on the residual representation.
We classify all mutation-finite quivers with real weights. We show that every finite mutation class not originating from an integer skew-symmetrisable matrix has a geometric realisation by reflections. We also explore the structure of acyclic representatives in finite mutation classes and their relations to acute-angled simplicial domains in the corresponding reflection groups.
The Ramsey number
$R(F,H)$
is the minimum number N such that any N-vertex graph either contains a copy of F or its complement contains H. Burr in 1981 proved a pleasingly general result that, for any graph H, provided n is sufficiently large, a natural lower bound construction gives the correct Ramsey number involving cycles:
$R(C_n,H)=(n-1)(\chi (H)-1)+\sigma (H)$
, where
$\sigma (H)$
is the minimum possible size of a colour class in a
$\chi (H)$
-colouring of H. Allen, Brightwell and Skokan conjectured that the same should be true already when
$n\geq \lvert H\rvert \chi (H)$
.
We improve this 40-year-old result of Burr by giving quantitative bounds of the form
$n\geq C\lvert H\rvert \log ^4\chi (H)$
, which is optimal up to the logarithmic factor. In particular, this proves a strengthening of the Allen–Brightwell–Skokan conjecture for all graphs H with large chromatic number.
We consider the Dirichlet Laplacian with uniform magnetic field on a curved strip in two dimensions. We give a sufficient condition on the width and the curvature of the strip ensuring the existence of the discrete spectrum in the strong magnetic field limit, answering (negatively) a conjecture made by Duclos and Exner.
We generalize the works of Pappas–Rapoport–Zhu on twisted affine Grassmannians to the wildly ramified case under mild assumptions. This rests on a construction of certain smooth affine
$\mathbb {Z}[t]$
-groups with connected fibers of parahoric type, motivated by previous work of Tits. The resulting
$\mathbb {F}_p(t)$
-groups are pseudo-reductive and sometimes non-standard in the sense of Conrad–Gabber–Prasad, and their
$\mathbb {F}_p [\hspace {-0,5mm}[ {t} ]\hspace {-0,5mm}] $
-models are parahoric in a generalized sense. We study their affine Grassmannians, proving normality of Schubert varieties and Zhu’s coherence theorem.
Let k be an algebraically closed field of prime characteristic p. Let
$kGe$
be a block of a group algebra of a finite group G, with normal defect group P and abelian
$p'$
inertial quotient L. Then we show that
$kGe$
is a matrix algebra over a quantised version of the group algebra of a semidirect product of P with a certain subgroup of L. To do this, we first examine the associated graded algebra, using a Jennings–Quillen style theorem.
As an example, we calculate the associated graded of the basic algebra of the nonprincipal block in the case of a semidirect product of an extraspecial p-group P of exponent p and order
$p^3$
with a quaternion group of order eight with the centre acting trivially. In the case of
$p=3$
, we give explicit generators and relations for the basic algebra as a quantised version of
$kP$
. As a second example, we give explicit generators and relations in the case of a group of shape
$2^{1+4}:3^{1+2}$
in characteristic two.
We define a notion of height for rational points with respect to a vector bundle on a proper algebraic stack with finite diagonal over a global field, which generalizes the usual notion for rational points on projective varieties. We explain how to compute this height for various stacks of interest (for instance: classifying stacks of finite groups, symmetric products of varieties, moduli stacks of abelian varieties, weighted projective spaces). In many cases, our uniform definition reproduces ways already in use for measuring the complexity of rational points, while in others it is something new. Finally, we formulate a conjecture about the number of rational points of bounded height (in our sense) on a stack
$\mathcal {X}$
, which specializes to the Batyrev–Manin conjecture when
$\mathcal {X}$
is a scheme and to Malle’s conjecture when
$\mathcal {X}$
is the classifying stack of a finite group.
Let G be a connected reductive group, T a maximal torus of G, N the normalizer of T and
$W=N/T$
the Weyl group of G. Let
${\mathfrak {g}}$
and
${\mathfrak {t}}$
be the Lie algebras of G and T. The affine variety
$\mathfrak {car}={\mathfrak {t}}/\!/W$
of semisimple G-orbits of
${\mathfrak {g}}$
has a natural stratification
indexed by the set of G-conjugacy classes of Levi subgroups: the open stratum is the set of regular semisimple orbits and the closed stratum is the set of central orbits.
In [17], Rider considered the triangulated subcategory
$D_{\mathrm {c}}^{\mathrm {b}}([{\mathfrak {g}}_{\mathrm {nil}}/G])^{\mathrm {Spr}}$
of
$D_{\mathrm {c}}^{\mathrm {b}}([{\mathfrak {g}}_{\mathrm {nil}}/G])$
generated by the direct summand of the Springer sheaf. She proved that it is equivalent to the derived category of finitely generated dg modules over the smash product algebra
${\overline {\mathbb {Q}}_{\ell }}[W]\# H^{\bullet }_G(G/B)$
where
$H^{\bullet }_G(G/B)$
is the G-equivariant cohomology of the flag variety. Notice that the later derived category is
$D_{\mathrm {c}}^{\mathrm {b}}(\mathrm {B}(N))$
where
$\mathrm {B}(N)=[\mathrm {Spec}(k)/N]$
is the classifying stack of N-torsors.
The aim of this paper is to understand geometrically and generalise Rider’s equivalence of categories: For each
$\lambda $
we construct a cohomological correspondence inducing an equivalence of categories between
$D_{\mathrm {c}}^{\mathrm {b}}([{\mathfrak {t}}_{\lambda }/N])$
and
$D_{\mathrm {c}}^{\mathrm {b}}([{\mathfrak {g}}_{\lambda }/G])^{\mathrm {Spr}}$
.
We propose a conjectural semiorthogonal decomposition for the derived category of the moduli space of stable rank 2 bundles with fixed determinant of odd degree, independently formulated by Narasimhan. We discuss some evidence for and furthermore propose semiorthogonal decompositions with additional structure.
We also discuss two other decompositions. One is a decomposition of this moduli space in the Grothendieck ring of varieties, which relates to various known motivic decompositions. The other is the critical value decomposition of a candidate mirror Landau–Ginzburg model given by graph potentials, which in turn is related under mirror symmetry to Muñoz’s decomposition of quantum cohomology. This corresponds to an orthogonal decomposition of the Fukaya category. We discuss how decompositions on different levels (derived category of coherent sheaves, Grothendieck ring of varieties, Fukaya category, quantum cohomology, critical sets of graph potentials) are related and support each other.
We study dynamics of solutions in the initial value space of the sixth Painlevé equation as the independent variable approaches zero. Our main results describe the repeller set, show that the number of poles and zeroes of general solutions is unbounded and that the complex limit set of each solution exists and is compact and connected.
Albert algebras, a specific kind of Jordan algebra, are naturally distinguished objects among commutative nonassociative algebras and also arise naturally in the context of simple affine group schemes of type
$\mathsf {F}_4$
,
$\mathsf {E}_6$
, or
$\mathsf {E}_7$
. We study these objects over an arbitrary base ring R, with particular attention to the case
$R = \mathbb {Z}$
. We prove in this generality results previously in the literature in the special case where R is a field of characteristic different from 2 and 3.
We prove several consistency results concerning the notion of
$\omega $
-strongly measurable cardinal in
$\operatorname {\mathrm {HOD}}$
. In particular, we show that is it consistent, relative to a large cardinal hypothesis weaker than
$o(\kappa ) = \kappa $
, that every successor of a regular cardinal is
$\omega $
-strongly measurable in
$\operatorname {\mathrm {HOD}}$
.
We introduce
$\varepsilon $
-approximate versions of the notion of a Euclidean vector bundle for
$\varepsilon \geq 0$
, which recover the classical notion of a Euclidean vector bundle when
$\varepsilon = 0$
. In particular, we study Čech cochains with coefficients in the orthogonal group that satisfy an approximate cocycle condition. We show that
$\varepsilon $
-approximate vector bundles can be used to represent classical vector bundles when
$\varepsilon> 0$
is sufficiently small. We also introduce distances between approximate vector bundles and use them to prove that sufficiently similar approximate vector bundles represent the same classical vector bundle. This gives a way of specifying vector bundles over finite simplicial complexes using a finite amount of data and also allows for some tolerance to noise when working with vector bundles in an applied setting. As an example, we prove a reconstruction theorem for vector bundles from finite samples. We give algorithms for the effective computation of low-dimensional characteristic classes of vector bundles directly from discrete and approximate representations and illustrate the usage of these algorithms with computational examples.