# Period preserving properties of an invariant from the permanent of signed incidence matrices

### Iain Crump

Simon Fraser University, Burnaby, Canada### Matt DeVos

Simon Fraser University, Burnaby, Canada### Karen Yeats

Simon Fraser University, Burnaby, Canada

## Abstract

A 4-point Feynman diagram in scalar $ϕ_{4}$ theory is represented by a graph $G$ which is obtained from a connected 4-regular graph by deleting a vertex. The associated Feynman integral gives a quantity called the period of $G$ which is invariant under a number of meaningful graph operations – namely, planar duality, the Schnetz twist, and it also does not depend on the choice of vertex which was deleted to form $G$.

In this article we study a graph invariant we call the graph permanent, which was implicitly introduced in a paper by Alon, Linial and Meshulam [1]. The graph permanent applies to any graph $G=(V,E)$ for which $∣E∣$ is a multiple of $∣V∣−1$ (so in particular to graphs obtained from a 4-regular graph by removing a vertex). We prove that the graph permanent, like the period, is invariant under planar duality and the Schnetz twist when these are valid operations, and we show that when $G$ is obtained from a $2k$-regular graph by deleting a vertex, the graph permanent does not depend on the choice of deleted vertex.

## Cite this article

Iain Crump, Matt DeVos, Karen Yeats, Period preserving properties of an invariant from the permanent of signed incidence matrices. Ann. Inst. Henri Poincaré Comb. Phys. Interact. 3 (2016), no. 4, pp. 429–454

DOI 10.4171/AIHPD/35