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Bounded-depth circuits cannot sample good codes. (English) Zbl 1282.68125

Summary: We study a variant of the classical circuit-lower-bound problems: proving lower bounds for sampling distributions given random bits. We prove a lower bound of \(1 - 1/n^{\Omega(1)}\) on the statistical distance between (i) the output distribution of any small constant-depth (a.k.a. \(\mathrm{AC}^0\)) circuit \(f:\{0, 1\}^{\mathrm{poly}(n)} \rightarrow \{0,1\}^n\), and (ii) the uniform distribution over any code \(\mathcal C \subseteq \{0, 1\}^n\) that is “good”, that is, has relative distance and rate both \(\Omega (1)\). This seems to be the first lower bound of this kind.
We give two simple applications of this result: (1) any data structure for storing codewords of a good code \(\mathcal C\subseteq \{0, 1\}^n\) requires redundancy \(\Omega (\log n)\), if each bit of the codeword can be retrieved by a small \(\mathrm{AC}^0\) circuit; and (2) for some choice of the underlying combinatorial designs, the output distribution of Nisan’s pseudorandom generator against \(\mathrm{AC}^0\) circuits of depth \(d\) cannot be sampled by small \(\mathrm{AC}^0\) circuits of depth less than \(d\).

MSC:

68Q17 Computational difficulty of problems (lower bounds, completeness, difficulty of approximation, etc.)
Full Text: DOI

References:

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