Another incompleteness result for Hoare's logic

It is known (Bergstra and Tucker (1982) J. Comput. System Sci. 25, 217) that if the Hoare rules are complete for a first-order structure %plane1D;49C;, then the set of partial correctness assertions true over %plane1D;49C; is recursive in the first-order theory of %plane1D;49C;. We show that the converse is not true. Namely, there is a first-order structure %plane1D;49E; such that the set of partial correctness assertions true over %plane1D;49E; is recursive in the theory of %plane1D;49E;, but the Hoare rules are not complete for %plane1D;49E;

Expressiveness and the completeness of Hoare's logic

Three theorems are proven which reconsider the completeness of Hoare's logic for the partial correctness of while-programs equipped with a first-order assertion language. The results are about the expressiveness of the assertion language and the role of specifications in completeness concerns for the logic: (1) expressiveness is not a necessary condition on a structure for its Hoare logic to be complete, (2) complete number theory is the only extension of Peano Arithmetic which yields a logically complete Hoare logic and (3) a computable structure with enumeration is expressive if and only if its Hoare logic is complete