PSEUDORANDOM GENERATORS FOR REGULAR BRANCHING PROGRAMS

摘要

We give new pseudorandom generators for regular read-once branching programs of small width. A branching program is regular if the in-degree of every vertex in it is either 0 or 2, except for the first layer. For every width d and length n, our pseudorandom generator uses a seed of length O((log d + loglogn + log(1/?)) log n) to produce n bits that cannot be distinguished from a uniformly random string by any regular width d length n read-once branching program, except with probability ?. We also give a result for general read-once branching programs, in the case that there are no vertices that are reached with small probability. We show that if a (possibly nonregular) branching program of length n and width d has the property that every vertex in the program is traversed with probability at least γ on a uniformly random input, then the error of the generator above is at most 2?/γ~2. Finally, we show that the set of all binary strings with less than d nonzero entries forms a hitting set for regular width d branching programs.