Noetherian not implies zero divisor in minimal prime

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Statement

It is possible to have a Noetherian ring with a zero divisor that is not contained in any minimal prime ideal.

Related facts

Proof

Let k be a field. Consider the ring R=k[x,y]/(x2,xy). We have:

  • R is an affine ring over a field, hence is Noetherian.
  • The element yR is a zero divisor, since xy=0.
  • The only minimal prime in R is the ideal generated by x: Since x is nilpotent, any prime ideal must contain x, and hence must contain the ideal generated by x. On the other hand, the ideal generated by x is prime, because the quotient is isomorphic to k[x,y]/(x2,xy,x)=k[x,y]/(x)k[y], which is an integral domain. Thus, (x) is the unique minimal prime.
  • y is not in the ideal generated by x: This is clear from the above remarks: the quotient R/(x) can naturally be identified with a polynomial ring in y, hence y is nonzero in the quotient, and not in (x).

Thus, we have a Noetherian ring with a zero divisor outside all minimal prime ideals.