科研之友
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摘要
We introduce the notion of functional extension of a set X, by means of two natural algebraic properties of the operator %26quot;(*)%26quot; on unary functions. We study the connections with ultrapowers of structures with universe X, and we give a simple characterization of those functional extensions that correspond to limit ultrapower extensions. In particular we obtain a purely algebraic proof of Keisler%26apos;s characterization of nonstandard (= complete elementary) extensions.
- 出版日期2012-8
