# Regular sequence

In commutative algebra, a regular sequence is a sequence of elements of a commutative ring which are as independent as possible, in a precise sense. This is the algebraic analogue of the geometric notion of a complete intersection.

## Definitions

Given a commutative ring R and an R-module M, an element r in R is called a non-zero-divisor on M if r m = 0 implies m = 0 for m in M. An M-regular sequence is a sequence r$_1$, ..., r$_d$ of elements of R such that r$_1$ is a not a zero-divisor on M and r$_i$ is a not a zero-divisor on M/(r$_1$, ..., r$_{i−1}$)M for i = 2, ..., d. Some authors also require that M/(r$_1$, ..., r$_d$)M is not zero. Intuitively, to say that r$_1$, ..., r$_d$ is an M-regular sequence means that these elements "cut M down" as much as possible, when we pass successively from M to M/(r$_1$)M, to M/(r$_1$, r$_2$)M, and so on.
An R-regular sequence is called simply a regular sequence. That is, r$_1$, ..., r$_d$ is a regular sequence if r$_1$ is a non-zero-divisor in R, r$_2$ is a non-zero-divisor in the ring R/(r$_1$), and so on. In geometric language, if X is an affine scheme and r$_1$, ..., r$_d$ is a regular sequence in the ring of regular functions on X, then we say that the closed subscheme {r$_1$=0, ..., r$_d$=0} ⊂ X is a complete intersection subscheme of X.
Being a regular sequence may depend on the order of the elements. For example, x, y(1-x), z(1-x) is a regular sequence in the polynomial ring C[x, y, z], while y(1-x), z(1-x), x is not a regular sequence. But if R is a Noetherian local ring and the elements r$_i$ are in the maximal ideal, or if R is a graded ring and the r$_i$ are homogeneous of positive degree, then any permutation of a regular sequence is a regular sequence.
Let R be a Noetherian ring, I an ideal in R, and M a finitely generated R-module. The depth of I on M, written depth$_R$(I, M) or just depth(I, M), is the supremum of the lengths of all M-regular sequences of elements of I. When R is a Noetherian local ring and M is a finitely generated R-module, the depth of M, written depth$_R$(M) or just depth(M), means depth$_R$(m, M); that is, it is the supremum of the lengths of all M-regular sequences in the maximal ideal m of R. In particular, the depth of a Noetherian local ring R means the depth of R as a R-module. That is, the depth of R is the maximum length of a regular sequence in the maximal ideal.
For a Noetherian local ring R, the depth of the zero module is ∞, whereas the depth of a nonzero finitely generated R-module M is at most the Krull dimension of M (also called the dimension of the support of M).

## Examples

* Given an integral domain $R$ any nonzero $f\in R$ gives a regular sequence.
* For a prime number p, the local ring Z$_{(p)}$ is the subring of the rational numbers consisting of fractions whose denominator is not a multiple of p. The element p is a non-zero-divisor in Z$_{(p)}$, and the quotient ring of Z$_{(p)}$ by the ideal generated by p is the field Z/(p). Therefore p cannot be extended to a longer regular sequence in the maximal ideal (p), and in fact the local ring Z$_{(p)}$ has depth 1.
* For any field k, the elements x$_1$, ..., x$_n$ in the polynomial ring A = k[x$_1$, ..., x$_n$] form a regular sequence. It follows that the localization R of A at the maximal ideal m = (x$_1$, ..., x$_n$) has depth at least n. In fact, R has depth equal to n; that is, there is no regular sequence in the maximal ideal of length greater than n.
* More generally, let R be a regular local ring with maximal ideal m. Then any elements r$_1$, ..., r$_d$ of m which map to a basis for m/m$^2$ as an R/m-vector space form a regular sequence.
An important case is when the depth of a local ring R is equal to its Krull dimension: R is then said to be Cohen-Macaulay. The three examples shown are all Cohen-Macaulay rings. Similarly, a finitely generated R-module M is said to be Cohen-Macaulay if its depth equals its dimension.

### Non-Examples

A simple non-example of a regular sequence is given by the sequence $(xy,x^{2})$ of elements in $\mathbb {C} [x,y]$ since
    $\cdot x^{2}:{\frac {\mathbb {C} [x,y]}{(xy)}}\to {\frac {\mathbb {C} [x,y]}{(xy)}}$
has a non-trivial kernel given by the ideal $(y)\subset \mathbb {C} [x,y]/(xy)$ . Similar examples can be found by looking at minimal generators for the ideals generated from reducible schemes with multiple components and taking the subscheme of a component, but fattened.

## Applications

* If r$_1$, ..., r$_d$ is a regular sequence in a ring R, then the Koszul complex is an explicit free resolution of R/(r$_1$, ..., r$_d$) as an R-module, of the form:
    $0\rightarrow R^{\binom {d}{d}}\rightarrow \cdots \rightarrow R^{\binom {d}{1}}\rightarrow R\rightarrow R/(r_{1},\ldots ,r_{d})\rightarrow 0$
In the special case where R is the polynomial ring k[r$_1$, ..., r$_d$], this gives a resolution of k as an R-module.
* If I is an ideal generated by a regular sequence in a ring R, then the associated graded ring
    $\oplus _{j\geq 0}I^{j}/I^{j+1}$
is isomorphic to the polynomial ring (R/I)[x$_1$, ..., x$_d$]. In geometric terms, it follows that a local complete intersection subscheme Y of any scheme X has a normal bundle which is a vector bundle, even though Y may be singular.