Beyond Bass Collapse: New Irregular Edge-Space Invariants in Ihara Theory

TL;DR

This paper introduces new invariants in Hashimoto--Ihara theory that distinguish irregular graphs beyond traditional spectral methods, using a detailed analysis of the edge-reversal symmetry and associated matrix factorizations.

Contribution

It identifies canonical edge-space invariants in Hashimoto theory that can differentiate irregular graphs with identical spectra, expanding the understanding of graph invariants.

Findings

Edge reversal induces a symmetric/antisymmetric splitting of the Hashimoto operator.

A determinant factorization isolates a correction sector revealing new graph invariants.

Irregular non-isomorphic graphs can be distinguished by these new invariants even when traditional spectra coincide.

Abstract

Let \(G\) be a finite simple graph and let \(T\) be its Hashimoto operator on the directed-edge space. We show that edge reversal induces a canonical symmetric/antisymmetric splitting under which \(T\) acquires an explicit \(2\times 2\) block form. The diagonal blocks are \(\tfrac12 L(G)\) and \(-\tfrac12 A(G)\), where \(L(G)\) is the line-graph adjacency and \(A(G)\) is the antisymmetric line-graph adjacency, while the off-diagonal block is the mixed incidence product \(M=|D|^\top D\). This identifies the ordinary and antisymmetric line-graph sectors as the two canonical diagonal sectors of Hashimoto theory and isolates a mixed sector linking them. A Schur-complement argument then gives a factorization \[ \det(I-wT)=\det\!\bigl(I-\tfrac w2 L(G)\bigr)\,C_G(w), \] where \(C_G(w)\) is an explicit correction determinant built from the antisymmetric and mixed sectors. We show that the trivial roots \(w=\pm1\) localize on canonical edge subspaces, and that for line-graph-cospectral pairs all remaining Ihara separation is forced into the correction sector. Although the raw mixed block \(M\) depends on edge orientation, its natural gauge-invariant shadows, including \(MM^\top\), \(M^\top M\), and \(M^\top L^kM\), define a canonical matrix package attached to the graph. In the regular case these collapse to adjacency-side data, but in the irregular case they need not. As an application, we exhibit irregular non-isomorphic graphs that are adjacency-cospectral and line-graph-cospectral yet are separated by the correction sector, and we find further examples where the gauge-invariant mixed shadows separate even when the scalar Ihara polynomial does not. This isolates new irregular edge-space invariants in Hashimoto--Ihara theory.