# Showing an Artinian ring, all of whose maximal ideals are principal, is a principal ideal ring.

Let $$R$$ be a commutative Artinian ring all of whose maximal ideals are principal. I wish to show that in fact all ideals are $$R$$ are principal.

As is outlined here Every maximal ideal is principal. Is $$R$$ principal?, a proof by Kaplansky gives this for Noetherian rings. This immediately answers my question.

Could we answer the question instead this way: By first showing that each local Artinian ring has this property, and then noting that each Artinian ring can be written as a product of Artinian local rings? These are Proposition 8.8 and Theorem 8.7 in Atiyah-Macdonald, respectively.

Suppose $$R \cong R_1 \times \cdots \times R_n$$ where $$R_i$$ are Artinian local rings. Then an ideal $$I \subset R$$ is of the form $$I_1 \times \cdots \times I_n \subset R_1 \times \cdots \times R_n$$ for some ideals $$I_i \subset R_i$$. By above, each $$I_i$$ is principal. So $$I_1 \times \cdots \times I_n$$ is principal $$\implies I$$ is principal.

Is this correct or did I do a dumb thing?

## 1 Answer

This is a valid reduction of the problem from commutative Artinian rings to commutative local Artinian rings.

Now... have you managed to do it in the local case yet?

Here is a different proof. Suppose there exists a nonprincipal ideal. Then there is a maximal nonprincipal ideal, which is prime. But prime ideals are maximal in Artinian rings, so it must be principal by our initial assumption, and that is a contradiction.

• Oh yes, I like your approach better. It takes some thought to show that each maximal ideal in $R_i$ is principal. Thoughts I don't want to think – Pentaki Sep 27 '18 at 18:29