Positivity conjecture for the Jacobian of the multigraph moment map

Let h:Rn{0}Rnh:\mathbb{R}^n\setminus\{\mathbf{0}\}\to\mathbb{R}^n be defined componentwise by

hi(β)=jiβiβj2ψβiβj.h_i(\boldsymbol{\beta})=\sum_{j\ne i}\frac{\beta_i\beta_j}{2\psi-\beta_i\beta_j}.

Let J\mathbf{J} be the Jacobian of hh, and let

B={ββ>0,  ββT>2ψE}.\mathcal{B}=\{\boldsymbol{\beta}\mid \boldsymbol{\beta}>\mathbf{0},\;\boldsymbol{\beta}\boldsymbol{\beta}^{T}>2\psi\mathbf{E}\}.

For βB\boldsymbol{\beta}\in\mathcal{B}, let λn(β)\lambda_n(\boldsymbol{\beta}) be the smallest eigenvalue of J(β)\mathbf{J}(\boldsymbol{\beta}). Positivity conjecture. For all βB\boldsymbol{\beta}\in\mathcal{B}, λn(β)>0\lambda_n(\boldsymbol{\beta})>0. In particular, J\mathbf{J} is positive definite and nonsingular. Furthermore, if β1\boldsymbol{\beta}\geq 1 entrywise, then λn(β)1\lambda_n(\boldsymbol{\beta})\geq 1. Numerical evidence suggests this conjecture, which would establish nonsingularity and positive definiteness of the Jacobian on the feasible set.

Sources & referencesView supporting material

Primary source

Philip S. Chodrow, “Moments of Uniform Random Multigraphs with Fixed Degree Sequences”, arXiv:1909.09037 (2020).

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