Weak Derandomization of Weak Algorithms: Explicit Versions of Yao's Lemma

• Authors: Ronen Shaltiel
• Venue: CCC '09 Proceedings of the 2009 24th Annual IEEE Conference on Computational Complexity
• Year: 2009

A simple averaging argument shows that given a randomized algorithm $A$ and a function $f$ such that for every input $x$, $\Pr[A(x)=f(x)] \ge 1-\rho$ (where the probability is over the coin tosses of $A$), there exists a \emph{nonuniform} deterministic algorithm $B$ ``of roughly the same complexity'' such that $\Pr[B(x)=f(x)] \ge 1-\rho$ (where the probability is over a uniformly chosen input $x$). This implication is often referred to as ``the easy direction of Yao's lemma'' and can be thought of as ``weak derandomization'' in the sense that $B$ is deterministic but only succeeds on most inputs. The implication follows as there exists a fixed value $r'$ for the random coins of $A$ such that ``hardwiring $r'$ into $A$'' produces a deterministic algorithm $B$. However, this argument does not give a way to \emph{explicitly construct} $B$. In this paper we consider the task of proving \emph{uniform versions} of the implication above. That is, how to \emph{explicitly construct} a deterministic algorithm $B$ when given a randomized algorithm $A$. We prove such derandomization results for several classes of randomized algorithms. These include: randomized communication protocols, randomized decision trees (here we improve a previous result by Zimand), randomized streaming algorithms and randomized algorithms computed by polynomial size constant depth circuits. Our proof uses an approach suggested by Goldreich and Wigderson and ``extracts randomness from the input''. We show that specialized (seedless) extractors can produce randomness that is in some sense not correlated with the input. Our analysis can be applied to \emph{any} class of randomized algorithms as long as one can explicitly construct the appropriate extractor. Some of our derandomization results follow by constructing a new notion of seedless extractors that we call ``extractors for recognizable distributions'' which may be of independent interest.