Alphabeta Math
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The ring (Z/2)N is zero-dimensional but not Noetherian

Example

Let R=(Z/2)N. Then every prime ideal of R is maximal, so R has Krull dimension 0, but R is not Noetherian.

Facts & Assumptions

Given: The ring R=(Z/2)N.

Verification

technique · direct
1.1givenalgebra

Every element x∈R is idempotent, because x2=x coordinatewise in Z/2. Let p be a prime ideal of R. Then R/P is an integral domain if and only if P is a prime ideal makes R/p an integral domain, and every class x‾∈R/p still satisfies x‾2=x‾. So x‾(x‾−1)=0 forces x‾=0 or x‾=1. Thus R/p has exactly two elements and is a field, so p is maximal.

2.1step 1.1givenalgebra

For each n≥0, let en be the sequence with 1 in coordinate n and 0 elsewhere, and let In:=Re0+⋯+Ren. Then I0⊊I1⊊I2⊊⋯ is a strict ascending chain of ideals, because en+1∈In+1∖In for every n. Therefore R is not Noetherian.

3.1step 1.1step 2.1givenalgebra∎

Let p0={x∈R:x0=0}. The first-coordinate projection R→Z/2 is a surjective ring homomorphism with kernel p0, so R/p0≅Z/2 is a field and therefore an integral domain. Thus R/P is an integral domain if and only if P is a prime ideal makes p0 a prime ideal. By step 1.1 every prime ideal of R is maximal, so no strict chain of prime ideals can have length greater than 0. Since p0 provides a prime ideal, chains of length 0 do occur. Therefore Krull dimension of a nonzero ring gives dim⁡R=0. Together with step 2.1, this ring is zero-dimensional but not Noetherian, so it is a concrete witness that the Noetherian hypothesis in the prime-maximal Artinian criterion cannot be dropped.

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Sources