DIMACS Theoretical Computer Science Seminar

Title: How Far Are We from Proving Circuit Size Lower Bounds?

Speaker: Eric Allender, Rutgers University

Date: Wednesday, October 3, 2007 11:00-12:00pm

Location: DIMACS Center, CoRE Bldg, Room 431, Rutgers University, Busch Campus, Piscataway, NJ


Many people are pessimistic about seeing a resolution to the P vs NP question any time soon. This pessimism extends also to questions about other important complexity classes, including two classes that will be the focus of this talk: TC^0 and NC^1.

TC^0 captures the complexity of several important computational problems, such as multiplication, division, and sorting; it consists of all problems computable by constant-depth, polynomial-size families of circuits of MAJORITY gates. TC^0_d is the subclass of TC^0 solvable with circuits of depth d. Although TC^0 seems to be a small subclass of P, it is still open if NP = TC^0_3.

NC^1 is the class of problems expressible by Boolean formulae of polynomial size. NC^1 contains TC^0, and captures the complexity of evaluating a Boolean formula. By a theorem of Barrington, NC^1 has a complete set that is regular: the word problem over S_5. (This is the problem of composing a sequence of permutations on {1,2,3,4,5} and determining if it evaluates to the identity permutation.)

Any proof that NP is not equal to TC^0 will have to overcome the obstacles identified by Razborov and Rudich in their paper on "Natural Proofs". That is, a "natural" proof that NP is not equal to TC^0 yields a proof that no pseudorandom function generator is computable in TC^0. This is problematical, since some popular cryptographic conjectures imply that such generators do exist. This leads to pessimism about the even more difficult task of separating NC^1 from TC^0.

Some limited lower bounds are within the grasp of current techniques, however. For example, several problems in P are known to require formulae of quadratic size -- but this seems to be of little use in trying to prove superpolynomial formula size. Along similar lines, we apply existing time-space tradeoff theorems to show that, for every d, there is a c>1 such that the standard complete set for NP^{SAT}requires uniform TC^0_d circuits of size at least n^c.

It might not seem too outrageous to hope to obtain a slightly stronger lower bound, showing that there is a c>1 such that this same set requires uniform TC^0 circuits of size n^c (regardless of the depth d). It also might not seem too outrageous to hope to obtain such a bound, even for the standard complete problem for NC^1 (the word problem over S_5). We show that this would be sufficient to prove that TC0 is properly contained in NC^1.

This is joint work with Michal Koucky.