Log-Submodularity of Zonoid Volume
Statement
The conjecture that volume is log-submodular under Minkowski addition on zonoids, that is |A||A+B+C| <= |A+B||A+C|. Disproved by a four-dimensional zonotope generated by a 2-modular matrix together with two segments. Several related local mixed-volume, local Loomis-Whitney, projection-volume-ratio and volume-to-surface-area conjectures fall with it.
Context
The paper also proves the conjecture in the unimodular case and characterizes equality there, so the boundary between true and false is drawn rather than just crossed.
A recent named conjecture in convex geometry whose failure also takes down several companion inequalities in the same programme.
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The paper has a section called How the counterexample was found. The author proved the unimodular case and identified 2-modular matrices as the place to look; then "Chat-GPT was then used to search for a counterexample within the space of the 2-modular matrices which led to the discovery of the counterexample within a specific class of 2-modular matrices". The acknowledgment is more conservative, crediting GPT-5.6 Pro with "literature searches and exploratory volume computations of 2-modular zonotopes", all independently verified by the author. Classified on the lower of the two readings.
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