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Leslie G. Valiant proved in 1979 that computing the permanent of a zero-one matrix is complete for the counting complexity class $\#\mathbf{P}$ under the paper's arithmetic reductions.^[ref]

For a bipartite graph's zero-one adjacency matrix, the permanent counts perfect matchings. Deciding whether one exists admits a polynomial-time algorithm, while counting all of them has the paper's much stronger completeness property. This makes the difference between existence and counting concrete. The permanent resembles the determinant but lacks its alternating signs, so an efficient determinant algorithm does not automatically compute it. Valiant's permanent reduction encodes logical constraints into weighted combinatorial choices.

Valiant's permanent reduction encodes satisfying assignments as weighted cycle covers of a directed graph. The permanent of a matrix sums the products of edge weights over these cycle covers of a directed graph.

Variable tracks enforce consistent truth values, clause interchanges enforce satisfaction, and signed junctions cancel invalid boundary choices. The original construction giveswhere $t(F)$ is twice the number of literal occurrences minus the number of clauses. Arithmetic recovers the predictable factor. The correspondence is between assignments and weighted groups of covers, rather than individual covers. Further transformations yield zero-one matrices.^[ref]