Abstract

We give syntactic characterizations of1. the theories whose categories of models are closed under the formation of pullbacks, and of2. the locally ω-polypresentable categories.A somewhat typical example is the category of algebraically closed fields. Case is proved by classical model-theoretic methods; it solves a problem raised by H. Volger . The solution of case is in the spirit of the ones for the locally ω-presentable and ω-multipresentable cases found by M. Coste and P.T. Johnstone respectively. The problem was raised in the context of Domain Theory by F. Lamarche