Abstract

The famous Gödel incompleteness theorem states that for every consistent, recursive, and sufficiently rich formal theory T there exist true statements that are unprovable in T . Such statements would be natural candidates for being added as axioms, but how can we obtain them? One classical approach is to add to some theory T an axiom that claims the consistency of T . In this paper we discuss another approach motivated by Chaitin's version of Gödel's theorem where axioms claiming the randomness of some strings are probabilistically added, and show that it is not really useful, in the sense that this does not help us prove new interesting theorems. This result answers a question recently asked by Lipton. The situation changes if we take into account the size of the proofs: randomly chosen axioms may help making proofs much shorter .We then study the axiomatic power of the statements of type “the Kolmogorov complexity of x exceeds n ” in general. They are Π1Π1 statements of Peano arithmetic. We show that by adding all true statements of this type, we obtain a theory that proves all true Π1Π1-statements, and also provide a more detailed classification. In particular, as Theorem 7 shows, to derive all true Π1Π1-statements it is enough to add one statement of this type for each n if strings are chosen in a special way. On the other hand, one may add statements of this type for most x of length n and still obtain a weak theory. We also study other logical questions related to “random axioms”.Finally, we consider a theory that claims Martin-Löf randomness of a given infinite binary sequence. This claim can be formalized in different ways. We show that different formalizations are closely related but not equivalent, and study their properties