algebra / Commutative algebra; homological conjectures

Hochster's small Cohen-Macaulay module conjecture for complete local domains

Theorem 1.1: there is a three-dimensional complete Noetherian normal local domain R⊃CR\supset\mathbb C with residue field C\mathbb C having no small Cohen-Macaulay module; the uncompleted vertex localization of the same section ring, essentially of finite type over C\mathbb C, also has none. The mechanism is Theorem 1.2: if the completed cone R(X,H)R(X,H) over a smooth projective surface has a small Cohen-Macaulay module, then KX2−2c2(X)≤6H2K_X^2-2c_2(X)\le6H^2, applied to a Hirzebruch-Kummer surface with KX≡5HK_X\equiv5H. It does not treat positive or mixed characteristic, the non-domain forms, or dimension three with other residue fields. Priority: Liang Chen posted a claimed counterexample of the same kind on 21 September 2026, two days before this manuscript (arXiv:2609.24142): the completed section ring of a degree-six Hirzebruch-Kummer surface with KY∼4HK_Y\sim4H, resting on his stated Main claim.

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algebraSep 23, 2026Significance 45/100Registry: unreviewed

Hochster's small Cohen-Macaulay module conjecture for complete local domains

Prior state unknown→disproved

Theorem 1.1: there is a three-dimensional complete Noetherian normal local domain R⊃CR\supset\mathbb C with residue field C\mathbb C having no small Cohen-Macaulay module; the uncompleted vertex localization of the same section ring, essentially of finite type over C\mathbb C, also has none. The mechanism is Theorem 1.2: if the completed cone R(X,H)R(X,H) over a smooth projective surface has a small Cohen-Macaulay modu…

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Theorem 1.1: there is a three-dimensional complete Noetherian normal local domain R⊃CR\supset\mathbb C with residue field C\mathbb C having no small Cohen-Macaulay module; the uncompleted vertex localization of the same section ring, essentially of finite type over C\mathbb C, also has none. The mechanism is Theorem 1.2: if the completed cone R(X,H)R(X,H) over a smooth projective surface has a small Cohen-Macaulay module, then KX2−2c2(X)≤6H2K_X^2-2c_2(X)\le6H^2, applied to a Hirzebruch-Kummer surface with KX≡5HK_X\equiv5H. It does not treat positive or mixed characteristic, the non-domain forms, or dimension three with other residue fields. Priority: Liang Chen posted a claimed counterexample of the same kind on 21 September 2026, two days before this manuscript (arXiv:2609.24142): the completed section ring of a degree-six Hirzebruch-Kummer surface with KY∼4HK_Y\sim4H, resting on his stated Main claim.

Theorem 1.1: there is a three-dimensional complete Noetherian normal local domain R⊃CR\supset\mathbb C with residue field C\mathbb C having no small Cohen-Macaulay module; the uncompleted vertex localization of the same section ring, essentially of finite type over C\mathbb C, also has none. The mechanism is Theorem 1.2: if the completed cone R(X,H)R(X,H) over a smooth projective surface has a small Cohen-Macaulay module, then KX2−2c2(X)≤6H2K_X^2-2c_2(X)\le6H^2, applied to a Hirzebruch-Kummer surface with KX≡5HK_X\equiv5H. It does not treat positive or mixed characteristic, the non-domain forms, or dimension three with other residue fields. Priority: Liang Chen posted a claimed counterexample of the same kind on 21 September 2026, two days before this manuscript (arXiv:2609.24142): the completed section ring of a degree-six Hirzebruch-Kummer surface with KY∼4HK_Y\sim4H, resting on his stated Main claim.

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