Height (ring theory)
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In commutative algebra, the height of an ideal I in a ring R is the number of strict inclusions in the longest chain of prime ideals contained in I. In the language of algebraic geometry, this is the codimension of the subvariety of Spec R corresponding to I.
It is not true that every chain of prime ideals contained in I has the same length; the first counterexample was found by Masayoshi Nagata. The existence of such an ideal is usually considered pathological and is ruled out by an assumption that the ring is catenary.
Many conditions on rings impose conditions on the heights of certain ideals or on all ideals of certain heights. Some notable conditions are:
- A ring is catenary if and only if for every two prime ideals P1 ⊆ P2, every saturated chain of strict inclusions P1 = Q0 ⊆ ... ⊆ Qh = P2 has the same length h.
- A ring is universally catenary if and only if any finitely generated algebra over it is catenary.
- A local ring is Cohen-Macaulay if and only if the height and depth of any two ideals are equal.
- A Noetherian ring is a unique factorization domain if and only if it is an integral domain and every height 1 prime ideal is principal.
In a Noetherian ring, Krull's height theorem says that the height of an ideal generated by n elements is no greater than n.
