12 research outputs found
The complexity of classification problems for models of arithmetic
We observe that the classification problem for countable models of arithmetic
is Borel complete. On the other hand, the classification problems for finitely
generated models of arithmetic and for recursively saturated models of
arithmetic are Borel; we investigate the precise complexity of each of these.
Finally, we show that the classification problem for pairs of recursively
saturated models and for automorphisms of a fixed recursively saturated model
are Borel complete.Comment: 15 page
Countable Short Recursively Saturated Models of Arithmetic
Short recursively saturated models of arithmetic are exactly the elementary initial segments of recursively saturated models of arithmetic. Since any countable recursively saturated model of arithmetic has continuum many elementary initial segments which are already recursively saturated, we turn our attention to the (countably many) initial segments which are not recursively saturated. We first look at properties of countable short recursively saturated models of arithmetic and show that although these models cannot be cofinally resplendent (an expandability property slightly weaker than resplendency), these models have non-definable expansions which are still short recursively saturated
Initial segments and end-extensions of models of arithmetic
This thesis is organized into two independent parts. In the first part, we extend the recent work on generic cuts by Kaye and the author. The focus here is the properties of the pairs (M, I) where I is a generic cut of a model M. Amongst other results, we characterize the theory of such pairs, and prove that they are existentially closed in a natural category. In the second part, we construct end-extensions of models of arithmetic that are at least as strong as ATR. Two new constructions are presented. The first one uses a variant of Fodor’s Lemma in ATR to build an internally rather classless model. The second one uses some weak versions of the Galvin–Prikry Theorem in adjoining an ideal set to a model of second-order arithmetic
A standard model of Peano arithmetic with no conservative elementary extension
AbstractThe principal result of this paper answers a long-standing question in the model theory of arithmetic [R. Kossak, J. Schmerl, The Structure of Models of Peano Arithmetic, Oxford University Press, 2006, Question 7] by showing that there exists an uncountable arithmetically closed family A of subsets of the set ω of natural numbers such that the expansion NA≔(N,A)A∈A of the standard model N≔(ω,+,×) of Peano arithmetic has no conservative elementary extension, i.e., for any elementary extension NA∗=(ω∗,…) of NA, there is a subset of ω∗ that is parametrically definable in NA∗ but whose intersection with ω is not a member of A. We also establish other results that highlight the role of countability in the model theory of arithmetic.Inspired by a recent question of Gitman and Hamkins, we furthermore show that the aforementioned family A can be arranged to further satisfy the curious property that forcing with the quotient Boolean algebra A/FIN (where FIN is the ideal of finite sets) collapses ℵ1 when viewed as a notion of forcing
Expansions, omitting types, and standard systems
Recursive saturation and resplendence are two important notions in models of
arithmetic. Kaye, Kossak, and Kotlarski introduced the notion of arithmetic
saturation and argued that recursive saturation might not be as rigid as first
assumed.
In this thesis we give further examples of variations of recursive
saturation, all of which are connected with expandability properties similar to
resplendence. However, the expandability properties are stronger than
resplendence and implies, in one way or another, that the expansion not only
satisfies a theory, but also omits a type. We conjecture that a special version
of this expandability is in fact equivalent to arithmetic saturation. We prove
that another of these properties is equivalent to \beta-saturation. We also
introduce a variant on recursive saturation which makes sense in the context of
a standard predicate, and which is equivalent to a certain amount of ordinary
saturation.
The theory of all models which omit a certain type p(x) is also investigated.
We define a proof system, which proves a sentence if and only if it is true in
all models omitting the type p(x). The complexity of such proof systems are
discussed and some explicit examples of theories and types with high
complexity, in a special sense, are given.
We end the thesis by a small comment on Scott's problem. We prove that, under
the assumption of Martin's axiom, every Scott set of cardinality <2^{\aleph_0}
closed under arithmetic comprehension which has the countable chain condition
is the standard system of some model of PA. However, we do not know if there
exists any such uncountable Scott sets.Comment: Doctoral thesi
On groups and initial segments in nonstandard models of Peano Arithmetic
This thesis concerns M-finite groups and a notion of discrete measure in models of Peano Arithmetic. First we look at a measure construction for arbitrary non-M-finite sets via suprema and infima of appropriate M-finite sets. The basic properties of the measures are covered, along with non-measurable sets and the use of end-extensions. Next we look at nonstandard finite permutations, introducing nonstandard symmetric and alternating groups. We show that the standard cut being strong is necessary and sufficient for coding of the cycle shape in the standard system to be equivalent to the cycle being contained within the external normal closure of the nonstandard symmetric group. Subsequently the normal subgroup structure of nonstandard symmetric and alternating groups is given as a result analogous to the result of Baer, Schreier and Ulam for infinite symmetric groups. The external structure of nonstandard cyclic groups of prime order is identified as that of infinite dimensional rational vector spaces and the normal subgroup structure of nonstandard projective special linear groups is given for models elementarily extending the standard model. Finally we discuss some applications of our measure to nonstandard finite groups