Is there a model theoretic realization of the concept of Arithmetical Hierachy? - MathOverflow

Is there a model theoretic realization of the concept of Arithmetical Hierachy?

2010-09-01T16:17:53Z

The question I want to ask is close to but not exactly what stated in the title:

Fix a language $L$, it is known that a statement $\sigma$ is universal in the language if whenever $M$ satisfies $\sigma $ and $N$ is a substructure of $M$ then $N$ also satisfy $\sigma$. It is also known that a statement $\sigma$ is existential if whenever $M$ satisfies $\sigma $ and $N$ is an extension of $M$ then $N$ also satisfy $\sigma$.

I can not find generalization of these criteria for formulas with more quantifiers. I wonder why this is the case?

Answer by Carl Mummert for Is there a model theoretic realization of the concept of Arithmetical Hierachy?

2010-09-01T17:48:11Z

There is one more well-known equivalence for $\forall \exists$ sentences.

Theorem (Chang-Los-Suszko). A theory $T$ is preserved under taking unions of increasing chains of structures if and only if $T$ is equivalent to a set of $\forall \exists$ sentences.

For a proof, see Keisler, "Fundamentals of model theory", Handbook of Mathematical Logic, p. 63.

I found a related paper, which is older and doesn't quite answer your question but may be of interest. R. C. Lyndon, "Properties preserved under algebraic constructions", Bull. Amer. Math. Soc. 65 n. 5 (1959), 287-299, Project Euclid

According to that paper, and MathSciNet, a general solution to your question should be contained in H. J. Keisler, "Theory of models with generalized atomic formulas", J. Symbolic Logic v. 25 (1960) 1-26, MathSciNet, JStor

Answer by Goldstern for Is there a model theoretic realization of the concept of Arithmetical Hierachy?

2012-01-08T23:16:19Z

You mean Keisler's sandwich theorem? Chang-Keisler, "Model Theory", theorem 5.2.8: A theory has a $\Pi_{2n}$-set of axioms if the class of its models is closed under $n$-sandwiches.