cs.LGJul 31, 2026

Pyramidal Width Can Increase Under Vertex Insertion

Authors: Jinze Zhao

Organizations: University of California, San Diego

Abstract

Lacoste-Julien and Jaggi conjectured in 2015 that the pyramidal width of a polytope cannot increase when a vertex is added, provided that every old point remains a vertex. We give an exact counterexample with six integer points in R3\R^3. For

P=\conv{v0,,v4},Q=\conv{v0,,v5},P=\conv\{v_0,\ldots,v_4\},\qquad Q=\conv\{v_0,\ldots,v_5\},

where

v0=(1,3,1),v1=(3,2,2),v2=(0,2,1),v3=(1,3,3),v4=(2,0,1),v5=(1,0,2),\begin{aligned} v_0&=(-1,-3,-1), & v_1&=(3,2,-2), & v_2&=(0,2,1),\\ v_3&=(-1,-3,3), & v_4&=(-2,0,1), & v_5&=(-1,0,-2), \end{aligned}

all five vertices of PP remain vertices of QQ, but

\PWidth(P)2=48353and\PWidth(Q)2=36133.\PWidth(P)^2=\frac{48}{353} \quad\text{and}\quad \PWidth(Q)^2=\frac{36}{133}.

Thus vertex insertion increases pyramidal width by the factor 1059/5321.410886779\sqrt{1059/532}\approx 1.410886779. The proof uses the equivalence between pyramidal width and facial distance, certifies both face lattices by integer supporting hyperplanes, and evaluates every facial distance by a finite rational calculation. A dependency-free exact verifier accompanies the paper.

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