# Why are maximal ideals prime?

Could anyone explain to me why maximal ideals are prime?

I'm approaching it like this, let $R$ be a commutative ring with $1$ and $A$ be a maximal ideal. Let $a,b\in R:ab\in A$

I'm trying to construct a ideal $B$ such that $A\subset B \neq A$ As this would be a contradiction. An alternative idea I had was to prove that $R/A$ is an integral domain, but this reduces to the same problem.

EDIT: Ergh.. just realized that I've learnt a theorem that states is $A$ is a maximal ideal then $R/A$ is a field

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Let $A$ be a maximal ideal. Then $R/A$ contains no proper ideals, by the correspondence theorem. Indeed, $R/A$ is a field (assuming that $R$ contains an identity).

Thm: R/A is a field.

Proof: Let $i+A\in R/A$ such that $i+A\neq 0+A$. We want to prove that $i+A$ is a unit. Then set $B=A+Ra=\{a+ri: a\in A, r\in R\}$.

Now, you (yourself!) need to prove that $B$ is an ideal, and that $A\subset B$ properly. So, since $A$ is maximal this means that $B=R$.

As $B=R$ we have that $1\in B$, so $(1+ri)+A=ri+A=(r+A)(i+A)$, and so $i+A$ is a unit, as required.

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 Thanks! That's a much neater proof than the one I have here.. will be making a note of your answer for writing up in my notes! :) – Freeman Sep 29 '11 at 11:06

Here’s a proof that doesn’t involve the quotient $R/A$.

Suppose that $A$ is not prime; then there are $a,b\in R\setminus A$ such that $ab\in A$. Let $B$ be the ideal generated by $A \cup \{a\}$; $B = \{x+ar: x\in A\text{ and }r\in R\}$. Clearly $A \subsetneq B$, so $B = R$, $1_R \in B$, and hence $1_R = x + ar$ for some $x\in A$ and $r\in R$. Then $$b = b1_R = b(x+ar) = bx + bar.$$ But $bx \in bA \subseteq RA = A$, and $bar \in Ar \subseteq AR = A$, so $b \in A$. This contradiction shows that $A$ is prime.

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 Thanks for showing me this, good to learn some less standard proofs – Freeman Sep 29 '11 at 11:15

$A$ is an maximal ideal $\Rightarrow$ $R/A$ is a field $\Rightarrow$ $R/A$ is an integral domain $\Rightarrow$ $A$ is prime

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@JyrkiLahtonen Haha thanks.. silly me. But I accepted the other guy to be nice. – Freeman Sep 29 '11 at 11:06
I suggested that you accept your own answer, 'cause I didn't see right away that Swlabr had already answered (I should be used to the delays by now). I deleted that comment as I felt that it was a tad rude at that point. I'm upvoting all the correct answers. – Jyrki Lahtonen Sep 29 '11 at 11:57