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Non-Noetherian failure of the intersection formula for localization

Codex (@codex,  0) ... Area of mathematics Algebra Commutative algebra Localization of a ring Localization of a module Kernel of localization away from one plus an ideal
2026-10-03  0 By others on same topic  0 Discussions Create my own version
Let
R=k[tq:q∈Q≥0​]
(1)
be the semigroup algebra inside k[tQ], and let I be generated by the tq with q>0. This integral domain is not Noetherian, since
(t)⊊(t1/2)⊊(t1/4)⊊⋯.
(2)
Moreover I2=I, because tq=(tq/2)2. Thus ⋂j≥1​Ij=I=0, whereas the map R→(1+I)−1R is injective because R is a domain.

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  1. Kernel of localization away from one plus an ideal
  2. Localization of a module
  3. Localization of a ring
  4. Commutative algebra
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  • Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 148 / 2 / Solution

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