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Extractors for polynomial sources over fields of constant order and small characteristic. (English) Zbl 1357.68148

Summary: A polynomial source of randomness over \(\mathbb F_q^n\) is a random variable \(X=f(Z)\) where \(f\) is a polynomial map and \(Z\) is a random variable distributed uniformly over \(\mathbb F_q^r\) for some integer \(r\). The three main parameters of interest associated with a polynomial source are the order \(q\) of the field, the (total) degree \(D\) of the map \(f\), and the base-\(q\) logarithm of the size of the range of \(f\) over inputs in \(\mathbb F_q^r\), denoted by \(k\). For simplicity we call \(X\) a \((q,D,k)\)-source. { } Informally, an extractor for \((q,D,k)\)-sources is a function \(E:\mathbb F_q^n\to \{0,1\}^m\) such that the distribution of the random variable \(E(X)\) is close to uniform over \(\{0,1\}^m\) for any \((q,D,k)\)-source \(X\). Generally speaking, the problem of constructing extractors for such sources becomes harder as \(q\) and \(k\) decrease and as \(D\) increases. A rather large number of recent works in the area of derandomization have dealt with the problem of constructing extractors for \((q,1,k)\)-sources, also known as “affine” sources. Constructing an extractor for non-affine sources, i.e., for \(D>1\), is a much harder problem. Prior to the present work, only one construction was known, and that construction works only for fields of order much larger than \(n\) [Z. Dvir et al., Comput. Complexity 18, No. 1, 1–58 (2009; Zbl 1210.68136) (Extended abstract appeared in FOCS 2007)]. In particular, even for \(D=2\), no construction was known for any fixed finite field. In this work we construct extractors for \((q,D,k)\)-sources for fields of constant order. Our proof builds on the work of M. DeVos and A. Gabizon, “Simple affine extractors using dimension expansion.” In: Proceedings of the 25th Annual IEEE Conference on Computational Complexity, p. 63 (2010)] on extractors for affine sources. Like the DeVos–Gabizon paper, our result makes crucial use of a theorem of Hou, Leung and Xiang [X.-D. Hou et al., J. Number Theory 97, No. 1, 1–9 (2002; Zbl 1034.11020)] which gives a lower bound on the dimension of products of subspaces.
The preliminary version has appeared in Lect. Notes Comput. Sci. 7408, 399–410 (2012; Zbl 1357.68147).

MSC:

68Q87 Probability in computer science (algorithm analysis, random structures, phase transitions, etc.)
11T23 Exponential sums
12E20 Finite fields (field-theoretic aspects)
60E15 Inequalities; stochastic orderings

References:

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