Positivity conjecture for the Jacobian of the multigraph moment map

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

hi(β)=∑j≠iβiβj2ψ−βiβj.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={β∣β>0,  ββT>2ψE}.\mathcal{B}=\{\boldsymbol{\beta}\mid \boldsymbol{\beta}>\mathbf{0},\;\boldsymbol{\beta}\boldsymbol{\beta}^{T}>2\psi\mathbf{E}\}.

For β∈B\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 β∈B\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 β≥1\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.

References

Primary source

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

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