Reduction in first-order logic compared with reduction in implicational logic

Abstract

In this paper we discuss strong normalization for natural deduction in the →∀-fragment of first-order logic. The method of collapsing types is used to transfer the result (concerning strong normalization) from implicational logic to first-order logic. The result is improved by a complement, which states that the length of any reduction sequence of derivation term r in first-order logic is equal to the length of the corresponding reduction sequence of its collapse term rc in implicational logic.