The Lattice Problem for Models of PA

The lattice problem for models of Peano Arithmetic (PA) is to determine which lattices can be represented as lattices of elementary submodels of a model of PA, or, in greater generality, for a given model M, which lattices can be represented as interstructure lattices of elementary submodels K of an elementary extension N such that M K N. The problem has been studied for the last 60 years and the results and their proofs show an interesting interplay between the model theory of PA, Ramsey style combinatorics, lattice representation theory, and some results in elementary number theory. We present a survey of the most important results together with a detailed analysis of some special cases to explain and motivate a technique of CPP-representations developed by James Schmerl for constructing elementary extensions with prescribed interstructure lattices. The last section is devoted to a discussion of lesser-known results about lattices of elementary submodels of countable recursively saturated models of PA.