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If T is an isol let D(T) be the least set of isols which contains T and is closed under predecessors and the application of almost recursive combinatorial functions. We find an infinite regressive isol T such that the universal theory (with respect to recursive relations and almost recursive combinatorial functions) of D(T) is the same as that of the nonnegative integers.
One of the earliest goals of modern logic was to characterize familiar mathematical structures up to isomorphism by means of properties expressed in a first order language. This hope was dashed by Skolem's discovery (cf. ) of a nonstandard model of first order arithmetic. A theory T such that any two of its models are isomorphic is called categorical. It is well known that if T has any infinite models then T is not categorical. We shall regain categoricity by
(i) enlarging our language so as to allow expressions of infinite length, and
(ii) enlarging our class of isomorphisms so as to allow isomorphisms existing in some Boolean valued extension of the universe of sets.
Let and be mathematical structures of the same similarity type where say R is binary on A. We write if f is an isomorphism of onto , and if there is an f such that . We say that P is a partial isomorphism of onto and write if P is a nonempty set of functions such that
(i) if f ∈ P then dom(f) is a substructure of , rng(/f) is a substructure of , and f is an isomorphism of its domain onto its range, and
(ii) if f ∈ P, a ∈ A, and b ∈ B then there exist g,h ∈ P, both extending f such that a ∈ dom(g) and b ∈ rng(h). Write if there is a P such that .
Let L be a first order logic and the infinitary logic (as described in [K, p. 6] over L. Suslin logic
is obtained from by adjoining new propositional operators and . Let f range over elements of ωω and n range over elements of ω. Seq is the set of all finite sequences of elements of ω. If θ: Seq → is a mapping into formulas of then and are formulas of LA. If is a structure in which we can interpret and h is an -assignment then we extend the notion of satisfaction from to by defining
where f ∣ n is the finite sequence consisting of the first n values of f. We assume that has ω symbols for relations, functions, constants, and ω1 variables. θ is valid if θ ⊧ [h] for every h and is valid if -valid for every . We address ourselves to the problem of finding syntactical rules (or nearly so) which characterize validity .
We examine the action of unary functions on the regressive isols. A manageable theory is produced and we find that such a function maps ⋀R into ⋀ if and only if it is eventually R↑ increasing and maps ⋀R into ⋀R if and only if it is eventually recursive increasing. Our paper concludes with a discussion of other methods for extending functions to ⋀R.
In this paper we show how nonrecursive combinatorial functions can be used to obtain another proof of the compactness Theorem 3.1 of . Our method is closely related to the argument used to show that a countable ultraproduct is ℵ1-saturated. What we do is to diagonalize in the most obvious way. The main difficulty with this approach is that the resulting diagonal function need not be recursive. Just to give an idea of how bad nonrecursive combinatorial functions are, we mention that by using frames (cf. ) they do not extend to Λ, and that by using normal combinatorial operators (cf. ) they do extend to Λ, map Λ into Λ, but in general, composition of such functions does not commute with their extension. We take care of this problem by constructing a large class of isols with respect to which the diagonal function is well behaved. The advantage of our method is that it provides the investigator with a natural and intuitive way of constructing counterexamples in his own research area.
In this paper we show (cf. Theorem 22) that in a language L* with equality, whose relation symbols denote arbitrary relations over ω* (=rational integers) and whose function symbols denote (= ∃∀ definable in the arithmetic hierarchy) functions over ω*, (i) a positive sentence is true in Λ* (= isolic integers) iff some Horn reduct is true in ω* with Skolem functions. We also show (cf Theorem 20) that (ii) a universally quantified sentence is true in Λ* iff some Horn reduct is true in Λ*.The latter result is nontrivial because our relations are arbitrary and our functions are In order to obtain (i) it was necessary to generalize the frame extensions of . This is done in §2. Our extension procedure agrees with that of  for recursive relations (cf. Theorem 12), and is certainly more general for − relations. What happens in the case is still open. In §3 we develop the basic properties of our extension so that in §4 we can prove a metatheorem (cf. Theorems 8 and 10) about Λ (=isols), in a language L with equality whose relation symbols denote arbitrary relations over ω (=nonnegative integers) and whose function symbols denote almost R↑ combinatorial functions. In Theorem 11 this is generalized to infinitary universal sentences. In §5 generic isols are introduced. These are used (cf. Theorems 16–19) to generalize and simplify the “fundamental lemma” of . The basic induction is patterned after Lemma 4.1 of , but is stronger in that any sufficiently generic assignment attainable from a frame yields Skolem functions. Finally in §6 these results are applied to Λ*, yielding the titled result (i) of our paper. Immediately following Theorem 15 there is a discussion which attempts to justify the way we extend relations to Λ.
In  it is shown that for every sequence x = 〈xn : n ∈ ω〉 ∈ Xω Λ there is an isol xω (essentially an immunized product) such that
Here we have used the notation: Λ = the isols, ω = the nonnegative integers, pn is the nth prime rational integer starting with p0 = 2, ∣ denotes divisibility and ∤ its negation. If p is an arbitrary prime, py ∣ x, pz ∣ x, and y < z then py+1 ∣ x. In particular since y ∈ ω is comparable with every element of Λ, the conditions py ∣ x and py+1 ∤ x uniquely determine y. Thus every sequence x ∈ Xωω is uniquely determined by an xω satisfying (1) and consequently may be used as a “code” for that sequence. In Theorem 1 it is shown that (1) does not uniquely determine the values of an arbitrary sequence x ∈ XωΛ, however in Theorem 3 we find a different scheme which does. At the very end of the paper we give some reasons why coding theorems are useful. It should also be mentioned that for a coding theorem to be meaningful it is necessary to restrict the operations by which a sequence can be recaptured from its code. Otherwise a triviality results. Our coding theorem will allow all operations which are first order definable in Λ with respect to addition, multiplication, and exponentiation. We conjecture that the latter operation is really necessary.
Let be a version of class set theory admitting urelemente, and with AC (= axiom of choice) replaced by AC0 (= axiom of choice for sets of finite sets), ω = nonnegative integers, and Δ = Dedekind cardinals. Let be an arbitrarily quantified positive first order sentence in functors for + and ·. Let ƒ0, … , ƒκ - 1 be function variables and the universal sentence obtained from by replacing existential quantifiers by the ƒ1 as Skolem functions.
In this paper we continue our investigation of the Dedekind cardinals which was initiated in . Those results are summarized below. Let ω be the finite cardinals and Δ the Dedekind cardinals. In  Myhill defined a class of functions ƒ: Χκω→ω, which he called the combinatorial functions, and which he applied to the study of recursive equivalence types.