Gödel's incompleteness theorem states that every finitely-presented, consistent, sound theory which is strong enough to include arithmetic is incomplete. In this paper we present elementary proofs for three axiomatic variants of Gödel's incompleteness theorem and we use them (a) to illustrate the idea that there is more than ``complete vs. incomplete", there are degrees of incompleteness, and (b) to discuss the implications of incompleteness and computer-assisted proofs for Hilbert's Programme. We argue that the impossibility of carrying out Hilbert's Programme is a thesis and has a similar status to the Church-Turing thesis.
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We undertake an axiomatic study of certain semirings and related structures that occur in operations research and computer science. We focus on the properties A,I,U,G,Z,L that have been used in the algebraic study of path problems in graphs and prove that the only implications linking the above properties are essentially those already known. On the other hand we extend those implications to the framework of left and right variants of A,I,U,G,Z,L, and we also prove two embedding theorems. Further generalizations refer mainly to semiring-like algebras with a partially defined addition, which is needed in semantics. As suggested by idempotency (I) and absorption (A), we also examine in some detail the connections between semirings and distributive lattices, which yield new systems of axioms for the latter.
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