Identifying an Honest ${\rm EXP}^{\rm NP}$ Oracle Among Many

We provide a general framework to remove short advice by formulating the following computational task for a function f : given two oracles at least one of which is honest (i.e. correctly computes f on all inputs) as well as an input, the task is to compute f on the input with the help of the oracles by a probabilistic polynomial-time machine, which we shall call a selector. We characterize the languages for which short advice can be removed by the notion of selector: a paddable language has a selector if and only if short advice of a probabilistic machine that accepts the language can be removed under any relativized world. Previously, instance checkers have served as a useful tool to remove short advice of probabilistic computation. We indicate that existence of instance checkers is a property stronger than that of removing short advice: although no instance checker for EXPNP-complete languages exists unless EXPNP = NEXP, we prove that there exists a selector for any EXPNP-complete language, by building on the proof of MIP = NEXP by Babai, Fortnow, and Lund (1991). 1998 ACM Subject Classification F.1.1 Models of Computation; F.1.2 Modes of Computation; F.1.3 Complexity Measures and Classes