Fast Odd-Permutation Sums in Characteristic Two and Shortest Even Directed Cycles

RESUMO

For a matrix $A$ over a field of characteristic two, let $\Phi(A)$ be the sum of its permutation monomials indexed by odd permutations. Although determinant and permanent coincide in this characteristic, this parity sub-sum retains information that neither gives separately. We show that $\Phi(A)$ and all its first partial derivatives can be computed deterministically in $O(n^\tau)$ field operations for every $n\times n$ matrix, where $2<\tau\le3$ is any fixed admissible matrix multiplication exponent. The result includes singular matrices and the binary field. An inversion-count identity expresses $\Phi$ through complementary minors. For invertible matrices, a decomposition along two binary interval trees aggregates these minors by matrix multiplication; a Boolean border of constant size handles the remaining ranks. We also give an explicit matrix formula for the full gradient. Applied to $\Phi(I+zW)$ for a randomly weighted adjacency matrix $W$, the evaluator computes the shortest even directed-cycle length in $\widetilde{O}(n^{\tau+1})$ bit operations, improving the $\widetilde{O}(n^{\tau+3})$ bound of Björklund, Husfeldt, and Kaski with the same multiplication exponent. The same time bound recovers the union of the arcs of all shortest even cycles with high probability, and deterministically outputs the cycle under a unique-shortest-cycle promise. For general graphs, exact maintenance of a nonzero coefficient gives an $\widetilde{O}(n^4)$ algorithm that outputs a shortest even cycle with high probability. Complementary extraction methods improve this bound for short cycles and for cycles that omit few vertices.