Semigroups, Horn sentences and isolic structures
Erik Ellentuck · Pacific Journal of Mathematics · 1975
We introduce a special type of semigroup whose properties are abstracted from the theory of cardinal numbers under addition.A very general theorem about the behavior of universal Horn sentences in such semigroups is obtained and then applied to isols, recursive equivalence types, cardinal numbers, and Dedekind cardinals, the last two being in set theory without the axiom of choice.