Pseudodeterministic constructions in subexponential time

We study pseudodeterministic constructions, i.e., randomized algorithms which output the same solution on most computation paths. We establish unconditionally that there is an infinite sequence {pn} of primes and a randomized algorithm A running in expected sub-exponential time such that for each n, on input 1|pn|, A outputs pn with probability 1. In other words, our result provides a pseudodeterministic construction of primes in sub-exponential time which works infinitely often. This result follows from a more general theorem about pseudodeterministic constructions. A property Q ⊆ {0,1}* is ϒ-dense if for large enough n, |Q ∩ {0,1}n| ≥ ϒ2n. We show that for each c > 0 at least one of the following holds: (1) There is a pseudodeterministic polynomial time construction of a family {Hn} of sets, Hn ⊆ {0,1}n, such that for each (1/nc)-dense property Q Ε DTIME(nc) and every large enough n, Hn ∩ Q ≠ ∅ or (2) There is a deterministic sub-exponential time construction of a family {H′n} of sets, H′n ∩ {0,1}n, such that for each (1/nc)-dense property Q Ε DTIME(nc) and for infinitely many values of n, H′n ∩ Q ≠ ∅. We provide further algorithmic applications that might be of independent interest. Perhaps intriguingly, while our main results are unconditional, they have a non-constructive element, arising from a sequence of applications of the hardness versus randomness paradigm.