Volume 15, Issue 3, Septiembre 2000
A Preservation Theorem for Equality-Free Horn Sentences
We prove the following preservation theorem for the Horn fragment of Equality-free Logic:
Theorem 0.1. For any sentence σ ϵ L, the following are equivalent:
i ) σ is preserved under Hs , Hs -1 and PR .
i i ) σ is logically equivalent to an equality-free Horn sentence.