Abstract:

We consider the following problem: for given \(n, M,\) produce a sequence \(X_1, X_2, . . . , X_n\) of bits that fools every linear test modulo \(M\). We present two constructions of generators for such sequences. For every constant prime power \(M\), the first construction has seed length \(O_M(\log(n/\epsilon))\), which is optimal up to the hidden constant. (A similar construction was independently discovered by Meka and Zuckerman [MZ]). The second construction works for every \(M, n,\) and has seed length \(O(\log n + \log(M/\epsilon) \log( M \log(1/\epsilon)))\).

The problem we study is a generalization of the problem of constructing small bias distributions [NN], which are solutions to the \(M=2\) case. We note that even for the case \(M=3\) the best previously known con- structions were generators fooling general bounded-space computations, and required \(O(\log^2 n)\) seed length.

For our first construction, we show how to employ recently constructed generators for sequences of elements of \(\mathbb{Z}_M\) that fool small-degree polynomials modulo \(M\). The most interesting technical component of our second construction is a variant of the derandomized graph squaring operation of [RV]. Our generalization handles a product of two distinct graphs with distinct bounds on their expansion. This is then used to produce pseudorandom walks where each step is taken on a different regular directed graph (rather than pseudorandom walks on a single regular directed graph as in [RTV, RV]).

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