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arXiv · 2610.00079

When Matchgate Base Collapse Fails: A Qutrit Trichotomy and Unbounded Exact Width

Chenghua Liu·Boning Meng
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Abstract

Holographic algorithms solve planar counting problems by encoding each value of a finite domain into several Boolean matchgate wires. Base collapse asks whether every exact representation can be compressed to a number of wires per edge bounded only by the domain size. Chen (STOC 2016) and Xia (STOC 2016) established broad positive collapse theorems under full-rank and related structural hypotheses. Together, these works left open whether a domain-only collapse bound survives when several locally deficient signatures must share one encoding. We resolve this open problem negatively, already on a three-state (qutrit) domain. For every $a\ge4$, we construct a five-label integer-valued qutrit language of maximum arity $a$ with an exact rational matchgate presentation, yet every exactly equivalent label-preserving presentation requires common width $Ω(2^{a/2}/a)$, even if its domain, base, tensors, and complex weights may all change. The construction avoids the usual local degeneracies, so the obstruction is genuinely simultaneous. Algebraic geometry then drives an exhaustive trichotomy explaining the boundary of collapse: the gadget closure either separates into rays, is Gaussian-mobile and collapses to width at most three, or is confined to a rigid plane-plus-ray flag containing our unbounded family. Pure-spinor geometry forces compression in the mobile case, while dimension bounds for matchgate varieties and Zariski avoidance place exact integer data outside every low-width algebraic image. Thus one geometric framework explains both why collapse occurs and why it can fail without bound.

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Chenghua Liu, Boning Meng. 2026-09-06. When Matchgate Base Collapse Fails: A Qutrit Trichotomy and Unbounded Exact Width. https://arxiv.org/abs/2610.00079

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