Leo Simons. A reduction in the number of independent axiom schemata for S4. Notre Dame journal of formal logic, vol. 3 (1962), pp. 256–258. - Bolesław Sobociński. An axiom-system for {K; N}-propositional calculus related to Simons' axiomatization of S3.Notre Dame journal of formal logic, vol. 3 (1962), pp. 206–208.
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