Nelson's Logic S
Besides the better-known Nelson logic (N3) and paraconsistent logic (N4), in 1959 David Nelson introduced, with motivations of realizability and constructibility, a logic called S. The logic S was originally presented by means of a calculus (crucially lacking the contraction rule) with infinitely many rule schemata and no semantics (other than the intended interpretation into Arithmetic.) We look here at the propositional fragment of S, showing that it is algebraizable (in fact, implicative), in the sense of Blok and Pigozzi, with respect to a variety of three-potent involutive residuated lattices. We thus introduce the first known algebraic semantics for S as well as a finite Hilbert-style calculus equivalent to Nelson's presentation; this also allows us to clarify the relation between S and the other two Nelson logics N3 and N4.