In a World of BPP=P

1. In a World of BPP=P Oded Goldreich Weizmann Institute of Science

2. Talk’s Outline The concrete contents and main message of this talk is thatBPP=P if and only if there exists suitable pseudorandom generators. It was known for decades that suitable pseudorandom generators imply BPP=P. The novelty is in the converse. More generally, we explore what follows if BPP=P. We shall start with a brief review of pseudorandom generators. Throughout the talk, BPP and P denote classes of promise problems.

3. Pseudorandom generators: a general paradigm The term “pseudorandom generator” (PRG) refers to a general paradigm with numerous incarnations, ranging fromgeneral-purpose PRGs (i.e., fooling any efficient observer) to special-purpose PRGs (e.g., pairwise independence PRGs). The common themes (and differences) relate to (1) amount of stretching, (2) notion of “looking random”, and (3) complexity of (deterministic) generation (or stretching). N.B.: In all cases the PRG itself is a deterministic algorithm. output sequence seed G

4. output sequence seed G Pseudorandom generators: canonical derandomizers The term “pseudorandom generator” refers to a general paradigm with numerous incarnations. The common themes (and differences) relate to (1) amount of stretching, (2) notion of “looking random”, and (3) complexity of (deterministic) generation (or stretching). For the purpose of derandomizing (e.g., BPP) it suffices to use PRGs that run in exponential-time (i.e., exponential in length of their input seeds). Their output should look random to linear-time observers (i.e., linear in length of PRG’s output).  canonical derandomizers.

5. Canonical derandomizers (recap and use) Def (canonical derandomizer):A PRG that run in exponential-time (i.e., exponential in length of its input seed) producing output that looks random to linear-time observers (i.e., linear in length of PRG’s output). THM: If there exist canonical derandomizers of exponential stretch, then BPP is in P. (Start with a linear-time randomized algorithm.)First, combine the randomized algorithm with the PRG to obtain a functionaly equivalent randomized algorithm of logarithmic randomness complexity. Note that this increases the running time by an exp(log) = poly term.Functional equivalence follows by indistinguishability! Next, use straightforward derandomization, introducing an overhead of exp(log) = poly factor.

6. Canonical derandomizers (recap. and more/detailed) Canonical derandomizers (PRGs) also come in several flavors. In all, generation time is exponential (in seed’s length);the small variations refer to the exact formulation of the pseudorandomness condition and to the stretch function. The most standard formulation refers to all (non-uniform) linear-size circuits. (That’s the one we used in prior slide.) Also standard is a uniform formulation: For any fixed polynomial p, no probabilistic p-time algorithm can distinguish the PRG’s output from a truly random string with gap greater than 1/p. We refer to this notion. Indeed, we shall focus on exponential stretch… (The PRG’s running time, in terms of its output length may be larger than p.)

7. Canonical derandomizers (the uniform version, a sanity check) Well-known: Using canonical derandomizers of exponential stretch we can effectively put BPP in P; that is,for every problem in BPP and every polynomial p, we obtain a deterministic poly-time algorithm such that no (prob.) p-time algorithm can find (except w. prob. 1/p) an input on which the deterministic algorithm errs. First, combine the randomized algorithm with the PRG to obtain an effectively equivalent randomized algorithm of logarithmic randomness complexity. Note: A p-time algorithm finding an error yields a p-time distinguisher! Then, use straightforward derandomization. NEW: We “reverse” the foregoing connection, showing that if BPP is effectively in P, then one can construct canonical derandomizers of exponential stretch.

8. Reversing the PRG-to-derandomization connection Assume (for simplicity) that BPP=P (rather than only effectively so). We construct canonical derandomizers of exponential stretch. Note that a random function of exponential stretch has the desired (p-time) pseudorandomness feature(w.r.t gap 1/p, we use a seed of length O(log p)). But we need an explicit (deterministic) construction. Idea: Just derandomize the above construction by using BPP=P. Problem: BPP=P refers to decision(al) problems, whereas we have at hand a construction problem (or a search problem). Solution:Reduce “BPP-search” problems to BPP, via a deterministic poly-time reduction that carefully implements the standard bit-by-bit process. (BPP as promise problem used here!)

9. A closer look at the construction (search) problem Recall: We assume that BPP=P, and construct canonical derandomizers of exponential stretch. The search problem at hand:Given 1n, find a set Sn of n-bit long strings such that any p(n)-time observer cannot distinguish a string selected uniformly in Sn from a totally random string. (W.r.t gap1/p(n), where Sn has sizepoly(p(n))=poly(n).) Note: validity of solutions can be checked in BPP.BPP-search  finding solutions in PPT + checking them in BPP. Reduce “BPP-search” problems to BPP, by extending the (current) solution prefix according to an estimate of the probability that a random extension of this prefix yields a valid solution. (The estimate is obtained via a query to a BPP oracle (of a promise type).)

10. Summary: canonical derandomizers are necessary (not merely sufficient) for placing BPP in P • THM (1st version of equivalence): The following are equiv. • For every polynomial p, BPP is p-effectively in P. • For every polynomial p, there exists a p-robust canonical derandomizer of exponential stretch. THM (2nd version of equivalence): BPP=P iff there exists atargetedcanonical derandomizer of exponential stretch. A problem is p-effectively solved by a function F if no probabilistic p-time algorithm can find an input on which F errs.A PRGs is p-robust if no probabilistic p-time algorithm can distinguish its output from a truly random one with gap greater than 1/p.Targeted  auxiliary-input PRG (same aux to the PRG and its test).

11. Reflections on our construction of canonical derandomizers. Recall: We assumed that BPP=P, and constructed canonical derandomizers of exponential stretch. The construction of a canonical derandomizer may amount to a fancy diagonalization argument, where the “fancy” aspect refers to the need to estimate the average behavior of machines. Indeed, we saw that the construction of a suitable setSnreduces to obtaining such estimates, which are easy to get from a BPP oracle. One lesson is that BPP=P is equivalent to the existence of canonical derandomizers of exponential stretch. Another lesson is that derandomization may be more related to diagonalization than to “hard” lower bounds…

12. Additional thoughts (or controversies) Shall we see BPP=P proved in our lifetime? The (only) negative evidence we have is that this would imply circuit lower bounds in NEXP [IKW01, KI03]. But recall that we do know that NEXP  P/poly if and only if NEXP  MA, so is this negative evidence not similar to saying that derandomizing MA [or BPP] implies a “lower bound” on computing NEXP [or EXP] by MA [or BPP]? Furthermore, maybe this indicates that such lower bounds are within reach (cf. Williams)? Some researchers attribute great importance to the difference between promise problems and “pure” decision problems. I have blurred this difference, and believe that whenever it exists we should consider the (general) promise problem version.

13. The End The slides of this talk are available at http://www.wisdom.weizmann.ac.il/~oded/T/bpp.ppt The paper is available at http://www.wisdom.weizmann.ac.il/~oded/p_bpp.html