Multidimensional exact classes, smooth approximation and bounded 4-types

Abstract

In connection with the work of Anscombe, Macpherson, Steinhorn and the present author in [1] we investigate the notion of a multidimensional exact class ($R$-mec), a special kind of multidimensional asymptotic class ($R$-mac) with measuring functions that yield the exact sizes of definable sets, not just approximations. We use results about smooth approximation [24] and Lie coordinatisation [14] to prove the following result (Theorem 4.6.4), as conjectured by Macpherson: For any countable language $\mathcal{L}$ and any positive integer $d$ the class $\mathcal{C}(\mathcal{L},d)$ of all finite $\mathcal{L}$-structures with at most $d$ 4-types is a polynomial exact class in $\mathcal{L}$, where a polynomial exact class is a multidimensional exact class with polynomial measuring functions.

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