Randomness and the density of hard problems

A language L is random with respect to a given complexity class C if for all ′ ∈ C L and ′ disagree on half of all strings. It is known that for any complexity class there are recursive languages that are random with respect to that class. Here it is shown that there are tight space and time hierarchies of random languages, and that EXPTIME contains P-isomorphism classes containing only languages that are random with respect to polynomial-time computations. The technique used is extended to show that for any constructible bound on time or space it is possible to deterministically generate binary sequences that appear random to all prediction algorithms subject to the given resource bound. Furthermore, the generation of such a sequence requires only slightly more resources than the given bound.