Creative Commons Attribution 3.0 Unported license
We study the Fourier spectrum of functions f : {0,1}^{mk} -> {-1,0,1} which can be written as a product of k Boolean functions f_i on disjoint m-bit inputs. We prove that for every positive integer d, sum_{S subseteq [mk]: |S|=d} |hat{f_S}| = O(min{m, sqrt{m log(2k)}})^d . Our upper bounds are tight up to a constant factor in the O(*). Our proof uses Schur-convexity, and builds on a new "level-d inequality" that bounds above sum_{|S|=d} hat{f_S}^2 for any [0,1]-valued function f in terms of its expectation, which may be of independent interest.
As a result, we construct pseudorandom generators for such functions with seed length O~(m + log(k/epsilon)), which is optimal up to polynomial factors in log m, log log k and log log(1/epsilon). Our generator in particular works for the well-studied class of combinatorial rectangles, where in addition we allow the bits to be read in any order. Even for this special case, previous generators have an extra O~(log(1/epsilon)) factor in their seed lengths.
We also extend our results to functions f_i whose range is [-1,1].
@InProceedings{lee:LIPIcs.CCC.2019.7,
author = {Lee, Chin Ho},
title = {{Fourier Bounds and Pseudorandom Generators for Product Tests}},
booktitle = {34th Computational Complexity Conference (CCC 2019)},
pages = {7:1--7:25},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-116-0},
ISSN = {1868-8969},
year = {2019},
volume = {137},
editor = {Shpilka, Amir},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CCC.2019.7},
URN = {urn:nbn:de:0030-drops-108296},
doi = {10.4230/LIPIcs.CCC.2019.7},
annote = {Keywords: bounded independence plus noise, Fourier spectrum, product test, pseudorandom generators}
}