In commutative algebra, a branch of mathematics, going up and going down are terms which refer to certain properties of chains of prime ideals in integral extensions.
The phrase going up refers to the case when a chain can be extended by "upward inclusion", while going down refers to the case when a chain can be extended by "downward inclusion".
The major results are the Cohen–Seidenberg theorems, which were proved by Irvin S. Cohen and Abraham Seidenberg. These are known as the going-up and going-down theorems.
Let A ⊆ B be an extension of commutative rings.
The going-up and going-down theorems give sufficient conditions for a chain of prime ideals in B, each member of which lies over members of a longer chain of prime ideals in A, to be able to be extended to the length of the chain of prime ideals in A.
First, we fix some terminology. If
ak{p}
ak{q}
ak{q}\capA=ak{p}
(note that
ak{q}\capA
ak{p}
ak{q}
ak{q}
ak{p}
ak{p}
ak{q}
The extension A ⊆ B is said to satisfy the incomparability property if whenever
ak{q}
ak{q}'
ak{p}
ak{q}
ak{q}'
ak{q}'
ak{q}
The ring extension A ⊆ B is said to satisfy the going-up property if whenever
ak{p}1\subseteqak{p}2\subseteq … … … \subseteqak{p}n
is a chain of prime ideals of A and
ak{q}1\subseteqak{q}2\subseteq … \subseteqak{q}m
is a chain of prime ideals of B with m < n and such that
ak{q}i
ak{p}i
ak{q}1\subseteqak{q}2\subseteq … \subseteqak{q}m\subseteq … \subseteqak{q}n
such that
ak{q}i
ak{p}i
In it is shown that if an extension A ⊆ B satisfies the going-up property, then it also satisfies the lying-over property.
The ring extension A ⊆ B is said to satisfy the going-down property if whenever
ak{p}1\supseteqak{p}2\supseteq … … … \supseteqak{p}n
is a chain of prime ideals of A and
ak{q}1\supseteqak{q}2\supseteq … \supseteqak{q}m
is a chain of prime ideals of B with m < n and such that
ak{q}i
ak{p}i
ak{q}1\supseteqak{q}2\supseteq … \supseteqak{q}m\supseteq … \supseteqak{q}n
such that
ak{q}i
ak{p}i
There is a generalization of the ring extension case with ring morphisms. Let f : A → B be a (unital) ring homomorphism so that B is a ring extension of f(A). Then f is said to satisfy the going-up property if the going-up property holds for f(A) in B.
Similarly, if B is a ring extension of f(A), then f is said to satisfy the going-down property if the going-down property holds for f(A) in B.
In the case of ordinary ring extensions such as A ⊆ B, the inclusion map is the pertinent map.
The usual statements of going-up and going-down theorems refer to a ring extension A ⊆ B:
There is another sufficient condition for the going-down property:
Proof:[2] Let p1 ⊆ p2 be prime ideals of A and let q2 be a prime ideal of B such that q2 ∩ A = p2. We wish to prove that there is a prime ideal q1 of B contained in q2 such that q1 ∩ A = p1. Since A ⊆ B is a flat extension of rings, it follows that Ap2 ⊆ Bq2 is a flat extension of rings. In fact, Ap2 ⊆ Bq2 is a faithfully flat extension of rings since the inclusion map Ap2 → Bq2 is a local homomorphism. Therefore, the induced map on spectra Spec(Bq2) → Spec(Ap2) is surjective and there exists a prime ideal of Bq2 that contracts to the prime ideal p1Ap2 of Ap2. The contraction of this prime ideal of Bq2 to B is a prime ideal q1 of B contained in q2 that contracts to p1. The proof is complete. Q.E.D.