We know that the existence of good pseudorandom generators (PRGs) does not only imply $P=BPP$, but also $PromiseP=PromiseBPP$.

Let us assume $PromiseP\ne PromiseBPP$. Then good PRGs do not exist. However it is still possible that $P=BPP$ holds. If good PRGs do not exist then does $P=BPP$ imply $P=NP$ or is there a possibility $P\neq NP$ still might be true?

Is it possible to have $P=BPP$ without good PRGs and with $P\neq NP$ and with $PromiseP=PromiseBPP$?

  • 5
    If $PromiseBPP \neq PromiseP$ then $P \neq NP$. Proof: If $P = NP$ then $PH$ collapses to $P$, so (due to approximate counting in $PH$) we can solve the problem "given a circuit, approximate its acceptance probability" in polynomial time. This in turn implies $PromiseBPP$ is in $PromiseP$ (because this circuit approximation problem is "complete" for $PromiseBPP$). – Ryan Williams Oct 26 at 17:54

To put this in complexity terms, could it be possible that P = BPP and EXP has polynomial-size circuits (so no general derandomization)? Relative to a generic oracle you do get P=BPP for all input lengths, and EXP in P/poly for infinitely many lengths, a partial answer to your query. I don't know an oracle where P = BPP and EXP is in P/poly, but I'd think it should be possible.

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