On Exponential-Time Hypotheses, Derandomization, and Circuit Lower Bounds: Extended Abstract
Lijie Chen, Ron D. Rothblum, Roei Tell, Eylon Yogev · 2020
The Exponential-Time Hypothesis (ETH) is a strengthening of the P ≠ NP conjecture, stating that 3-SAT on n variables cannot be solved in (uniform) time 2ε·n, for some . In recent years, analogous hypotheses that are “exponentially-strong” forms of other classical complexity conjectures (such as NP ⊄ eq BPP or coNP ⊄ eq NP) have also been introduced, and have become widely influential. In this work, we focus on the interaction of exponential-time hypotheses with the fundamental and closely-related questions of derandomization and circuit lower bounds. We show that even relatively-mild variants of exponential-time hypotheses have far-reaching implications to derandomization, circuit lower bounds, and the connections between the two. Specifically, we prove that: 1) The Randomized Exponential-Time Hypothesis (rETH) implies that BPP can be simulated on “average-case” in deterministic (nearly-)polynomial-time (i.e., in time 2~O(log(n))=nloglog(n)O(1)). The derandomization relies on a conditional construction of a pseudorandom generator with near-exponential stretch (i.e., with seed length ~O(log(n))); this significantly improves the state-of-the-art in uniform “hardness-to-randomness” results, which previously only yielded pseudorandom generators with sub-exponential stretch from such hypotheses. 2) The Non-Deterministic Exponential-Time Hypothesis (NETH) implies that derandomization of BPP is completely equivalent to circuit lower bounds against E, and in particular that pseudorandom generators are necessary for derandomization. In fact, we show that the foregoing equivalence follows from a very weak version of NETH, and we also show that this very weak version is necessary to prove a slightly stronger conclusion that we deduce from it. Lastly, we show that disproving certain exponential-time hypotheses requires proving breakthrough circuit lower bounds. In particular, if CireuitSAT for circuits over n bits of size poly(n) can be solved by probabilistic algorithms in time 2n/polylog(n), then BPε does not have circuits of quasilinear size.