Concerning Keisler Measures over ultraproducts

Abstract

As consequence of the VC theorem, any pseudo-finite measure over an NIP ultraproduct is generically stable. We demonstrate a converse of this theorem and prove that any finitely approximable measure over an ultraproduct is itself pseudo-finite (even without the NIP assumption). We also analyze the connection between the Morley product and the pseudo-finite product. In particular, we show that if $\mu$ is definable and both $\mu$ and $\nu$ are pseudo-finite, then the Morley product of $\mu$ and $\nu$ agrees with the pseudo-finite product of $\mu$ and $\nu$. Using this observation, we construct generically stable idempotent measures on pseudo-finite NIP groups.