Modulating flow polynomials and parity bias in homological walk counting

Abstract

Long weighted walks seem to sample mod-$t$ flows uniformly, but at even moduli a persistent parity bias survives. Writing $C_G(t)$ for the flow polynomial of a connected graph $G$ of genus $g$, the proportion of nowhere-zero flows under uniform sampling is $C_G(t)/t^g$. For arbitrary positive directed weights and all moduli, we determine the exact leading corrections for both closed walks and nonbacktracking closed walks, which we call circuits. Only the circuit result requires $g\ge2$. For closed walks, the canonical character governs the leading correction through the canonical modulating flow polynomial $D_G$; for circuits, the period character, when it exists, governs the leading correction through the period modulating flow polynomial $D_{G,η}$. At even moduli these give parity-dependent corrections: the two mechanisms coincide on nonbipartite graphs and separate only in the bipartite circuit problem. More generally, each integral-homology character $ζ$ determines a balanced-dichromatic specialization $D_{G,ζ}$. These polynomials form a character-indexed family whose finite-order members give Fourier coefficients of the nowhere-zero flow set and whose retained edge sets are the central sets of a gain semimatroid. We further show that $D_G$ is a cycle-matroid invariant and a signed-Tutte specialization satisfying signed deletion-contraction. Finally, an integral nowhere-zero approximation theorem implies that asymptotically almost every weighted circuit traverses every edge.