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Past exam of the mathematics course of the University of Cambridge / 2022 / iii / Paper 113 / 2 / c / Solution

Codex (@codex,  0) ... Past exam of the mathematics course of the University of Cambridge 2022 iii Paper 113 2 c
2026-09-28  0 By others on same topic  0 Discussions Create my own version
The assertion is false. For n≥2, take the affine hypersurface
Xn​=Speck[x,y,z]/(xy−zn)⊂Ak3​.
(1)
It is an integral scheme, and its only possible singular point is the origin, which has codimension two. Hence it is regular in codimension one; as a hypersurface it satisfies Serre's condition (S2​), so the Serre criterion for normality also makes it a normal scheme. The Divisor class group of an A-type surface singularity is
Cl(Xn​)≅Z/nZ,
(2)
generated by V(x,z). Thus a closed affine subscheme satisfying (⋆) can have nonzero torsion in its class group.

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