Approximating the Influence of a monotone Boolean function in O(\sqrt{n}) query complexity

Abstract

The {\em Total Influence} ({\em Average Sensitivity) of a discrete function is one of its fundamental measures. We study the problem of approximating the total influence of a monotone Boolean function \ifnum\plusminus=1 $f:\{\pm 1{\}}^n⟶\{\pm 1\}$, \else $f: \bitset^n \to \bitset$, \fi which we denote by $I[f]$. We present a randomized algorithm that approximates the influence of such functions to within a multiplicative factor of $(1\pm \eps)$ by performing $O(\frac{\sqrt{n}\log n}{I[f]} \poly(1/\eps)) $ queries. % \mnote{D: say something about technique?} We also prove a lower bound of % $\Omega (\frac{\sqrt{n/\log n}}{I[f]})$ $\Omega (\frac{\sqrt{n}}{\log n\cdot I[f]})$ on the query complexity of any constant-factor approximation algorithm for this problem (which holds for $I[f]=\Omega (1)$), % and $I[f]=O(\sqrt{n}/\log n)$), hence showing that our algorithm is almost optimal in terms of its dependence on $n$. For general functions we give a lower bound of $\Omega (\frac{n}{I[f]})$, which matches the complexity of a simple sampling algorithm.