Sampling graphs without forbidden subgraphs and unbalanced expanders with negligible error

Suppose that you wish to sample a random graph G over n vertices and m edges conditioned on the event that G does not contain a 'small' t-size graph H (e.g., clique) as a subgraph. Assuming that most such graphs are H-free, the problem can be solved by a simple rejected-sampling algorithm (that tests for t-cliques) with an expected running time of nO(t). Is it possible to solve the problem in running time that does not grow polynomially with nt? In this paper, we introduce the general problem of sampling a 'random looking'' graph G with a given edge density that avoids some arbitrary predefined t-size subgraph H. As our main result, we show that the problem is solvable with respect to some specially crafted k-wise independent distribution over graphs. That is, we design a sampling algorithm for k-wise independent graphs that supports efficient testing for subgraph-freeness in time f(t)⋅ nc where f is a function of t and the constant c in the exponent is independent of t. Our solution extends to the case where both G and H are d-uniform hypergraphs. We use these algorithms to obtain the first probabilistic construction of constant-degree polynomially-unbalanced expander graphs whose failure probability is negligible in n (i.e., n-ω(1)). In particular, given constants d>c, we output a bipartite graph that has n left nodes, nc right nodes with right-degree of d so that any right set of size at most nΩ(1) expands by factor of Ω(d). This result is extended to the setting of unique expansion as well. We observe that such a negligible-error construction can be employed in many useful settings, and present applications in coding theory (batch codes and LDPC codes), pseudorandomness (low-bias generators and randomness extractors) and cryptography. Notably, we show that our constructions yield a collection of polynomial-stretch locally-computable cryptographic pseudorandom generators based on Goldreich's one-wayness assumption resolving a central open problem in parallel-cryptography (cf., Applebaum-Ishai-Kushilevitz, FOCS 2004; and Ishai-Kushilevitz-Ostrovsky-Sahai, STOC 2008).