Some Questions of Uniformity in Algorithmic Randomness

Abstract

The $\Omega$ numbers-the halting probabilities of universal prefix-free machines-are known to be exactly the Martin-L{ö}f random left-c.e. reals. We show that one cannot uniformly produce, from a Martin-L{ö}f random left-c.e. real $\alpha$, a universal prefix-free machine U whose halting probability is $\alpha$. We also answer a question of Barmpalias and Lewis-Pye by showing that given a left-c.e. real $\alpha$, one cannot uniformly produce a left-c.e. real $\beta$ such that $\alpha$ -- $\beta$ is neither left-c.e. nor right-c.e.