Range Avoidance for Constant-Depth Circuits: Hardness and Algorithms
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Range Avoidance (AVOID) is a total search problem where, given a Boolean circuit C{0,1}^n→{0,1}^m, m>n, the task is to find a y∈{0,1}^m outside the range of C. For an integer k≥ 2, NC^0_k-AVOID is a special case of AVOID where each output bit of C depends on at most k input bits. While there is a very natural randomized algorithm for AVOID, a deterministic algorithm for the problem would have many interesting consequences. Ren, Santhanam, and Wang (FOCS 2022) and Guruswami, Lyu, and Wang (RANDOM 2022) proved that explicit constructions of functions of high formula complexity, rigid matrices, and optimal linear codes, reduce to NC^0_4-AVOID, thus establishing conditional hardness of the NC^0_4-AVOID problem. On the other hand, NC^0_2-AVOID admits polynomial-time algorithms, leaving the question about the complexity of NC^0_3-AVOID open. We give the first reduction of an explicit construction question to NC^0_3-AVOID. Specifically, we prove that a polynomial-time algorithm (with an NP oracle) for NC^0_3-AVOID for the case of m=n+n^2/3 would imply an explicit construction of a rigid matrix, and, thus, a super-linear lower bound on the size of log-depth circuits. We also give deterministic polynomial-time algorithms for all NC^0_k-AVOID problems for m≥ n^k-1/log(n). Prior work required an NP oracle, and required larger stretch, m ≥ n^k-1.
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