Randomness and Semi-measures

Abstract

A semi-measure is a generalization of a probability measure obtained by relaxing the additivity requirement to super-additivity. We introduce and study several randomness notions for left-c.e. semi-measures, a natural class of effectively approximable semi-measures induced by Turing functionals. Among the randomness notions we consider, the generalization of weak 2-randomness to left-c.e. semi-measures is the most compelling, as it best reflects Martin-L\"of randomness with respect to a computable measure. Additionally, we analyze a question of Shen, a positive answer to which would also have yielded a reasonable randomness notion for left-c.e. semi-measures. Unfortunately though, we find a negative answer, except for some special cases.