Deterministic Approximation of Random Walks via Queries in Graphs of Unbounded Size

Consider the following computational problem: given a regular digraph G = (V,E), two vertices u, v ∈ V , and a walk length t ∈ N, estimate the probability that a random walk of length t from u ends at v to within ±ε. A randomized algorithm can solve this problem by carrying out O(1/ε) random walks of length t from u and outputting the fraction that end at v. In this paper, we study deterministic algorithms for this problem that are also restricted to carrying out walks of length t from u and seeing which ones end at v. Specifically, if G is d-regular, the algorithm is given oracle access to a function f : [d] → {0, 1} where f(x) is 1 if the walk from u specified by the edge labels in x ends at v. We assume that G is consistently labelled, meaning that the edges of label i for each i ∈ [d] form a permutation on V . We show that there exists a deterministic algorithm that makes poly(dt/ε) nonadaptive queries to f , regardless of the number of vertices in the graph G. Crucially, and in contrast to the randomized algorithm, our algorithm does not simply output the average value of its queries. Indeed, Hoza, Pyne, and Vadhan (ITCS 2021) showed that any deterministic algorithm of the latter form that works for graphs of unbounded size must have query complexity at least exp(Ω̃(log(t) log(1/ε))). In the language of pseudorandomness, our result is a separation between the query complexity of “deterministic samplers” and “deterministic averaging samplers” for the class of “permutation branching programs of unbounded width”. Our separation is stronger than the prior separation of Pyne and Vadhan (CCC 2021), and has a much simpler proof (not using spectral graph theory or the Impagliazzo–Nisan–Wigderson pseudorandom generator). On the other hand, the algorithm of Pyne and Vadhan is explicit and computable in small space, whereas ours is not explicit (unless we assume the existence of an optimal explicit pseudorandom generator for permutation branching programs of bounded width).