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Published online by Cambridge University Press: 12 March 2014
In his paper [3] Henkin proved for a first order language with identity symbol but without operation symbols the following version of the Craig-Lyndon interpolation theorem:
Theorem 1. If Γ╞Δ then there is a formula θ such that Γ ├Δand
(i) any relation symbol with a positive (negative) occurrence in θ has a positive (negative) occurrence in some formula of Γ.
The result is announced in [6]. It was obtained in fall 1966 and the paper read in the mathematical colloquium at Hannover (Germany) on November 25, 1966.