# Krull's theorem

In mathematics, and more specifically in ring theory, Krull's theorem, named after Wolfgang Krull, asserts that a nonzero ring has at least one maximal ideal. The theorem was proved in 1929 by Krull, who used transfinite induction. The theorem admits a simple proof using Zorn's lemma, and in fact is equivalent to Zorn's lemma, which in turn is equivalent to the axiom of choice.

## Variants

• For noncommutative rings, the analogues for maximal left ideals and maximal right ideals also hold.
• For pseudo-rings, the theorem holds for regular ideals.
• A slightly stronger (but equivalent) result, which can be proved in a similar fashion, is as follows:
Let R be a ring, and let I be a proper ideal of R. Then there is a maximal ideal of R containing I.
This result implies the original theorem, by taking I to be the zero ideal (0). Conversely, applying the original theorem to R/I leads to this result.
To prove the stronger result directly, consider the set S of all proper ideals of R containing I. The set S is nonempty since IS. Furthermore, for any chain T of S, the union of the ideals in T is an ideal J, and a union of ideals not containing 1 does not contain 1, so JS. By Zorn's lemma, S has a maximal element M. This M is a maximal ideal containing I.

## Krull's Hauptidealsatz

Another theorem commonly referred to as Krull's theorem:

Let $R$ be a Noetherian ring and $a$ an element of $R$ which is neither a zero divisor nor a unit. Then every minimal prime ideal $P$ containing $a$ has height 1.
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