Multidimensional asymptotic classes

Abstract

We develop a general framework (multidimensional asymptotic classes, or m.a.c.s) for handling classes of finite first order structures with a strong uniformity condition on cardinalities of definable sets: The condition asserts that definable families given by a formula \phi(x,y) should take on a fixed number n_\phi of approximate sizes in any M in the class, with those sizes varying with M. The prototype is the class of all finite fields, where the uniformity is given by a theorem of Chatzidakis, van den Dries and Macintyre. It inspired the development of asymptotic classes of finite structures, which this new framework extends. The underlying theory of m.a.c.s is developed, including preservation under bi-interpretability, and a proof that for the m.a.c. condition to hold it suffices to consider formulas \phi(x,y) with x a single variable. Many examples of m.a.c.s are given, including 2-sorted structures (F,V) where V is a vector space over a finite field F possibly equipped with a bilinear form, and an example arising from representations of quivers of finite representation type. We also give examples and structural results for multidimensional exact classes (m.e.c.s), where the definable sets take a fixed number of precisely specified cardinalities, which again vary with M. We also develop a notion of infinite generalised measurable structure, whereby definable sets are assigned values in an ordered semiring. We show that any infinite ultraproduct of a m.a.c. is generalised measurable, that values can be taken in an ordered ring if the m.a.c. is a m.e.c., and explore model-theoretic consequences of generalised measurability. Such a structure cannot have the strict order property, and stability-theoretic properties can be read off from the measures in the semiring.