Tom Kern

Ph.D. (2016) Cornell University


Nonstandard models of the weak second order theory of one successor


Anil Nerode


In this paper, we seek to classify the nonstandard models of the weak (monadic) second order theory of one successor (WS1S), the theory of the two-typed structure consisting of natural numbers and finite sets of natural number equipped with a successor (+1) operation, and membership relation between the two types. This will require an automata-theoretic approach. Chapter 1 will provide an introductory background to the intersection of automata theory and model theory. In chapter 2, we use the Krohn-Rhodes Theorem and an observation about the powerset determinization construction to prove what will be essentially be a quantifier-elimination type result for our theory, although since the result is of independent interest, it will be presented in a more general automata-theoretic context as a generating set for the regular functions under composition. In chapter 3, we provide an axiomatization for WS1S. In chapter 4, we apply this axiomatization to prove a number of key theorems regarding nonstandard models of WS1S. This includes a classification of the possible first order types, tools for completely classifying nonstandard models, an exploration of countable nonstandard models with the simplest nontrivial first order type, and a tool for producing new nonstandard models given old nonstandard models.