Hilbert's syzygy theorem

In mathematics, Hilbert's syzygy theorem is one of the three fundamental theorems about polynomial rings over fields, first proved by David Hilbert in 1890, which were introduced for solving important open questions in invariant theory, and are at the basis of modern algebraic geometry. The two other theorems are Hilbert's basis theorem that asserts that polynomial rings are Noetherian, and Hilbert's Nullstellensatz, which establishes a bijective correspondence between affine algebraic varieties and prime ideals of polynomial rings.

Hilbert's syzygy theorem concern the relations, or syzygies in Hilbert's terminology, between the generators of an ideal, or, more generally, a module. As the relations form a module, one may consider the relations between the relations; Hilbert's syzygy theorem asserts that, if one continues in this way, starting with a module over a polynomial ring in n indeterminates over a field, one eventually finds a zero module of relations, after at most n steps.

Hilbert's syzygy theorem is now considered to be an early result of homological algebra. It is the starting point of the use of homological methods in commutative algebra and algebraic geometry.

History

The syzygy theorem first appeared in Hilbert's seminal paper "Über die Theorie der algebraischen Formen" (1890).[1] The paper is split into five parts: part I proves Hilbert's basis theorem over a field, while part II proves it over the integers. Part III contains the syzygy theorem (Theorem III), which is used in part IV to discuss the Hilbert polynomial. The last part, part V, proves finite generation of certain rings of invariants. Incidentally part III also contains a special case of the Hilbert–Burch theorem.

Syzygies (relations)

Originally, Hilbert defined syzygies for ideals in polynomial rings, but the concept generalizes trivially to (left) modules over any ring.

Given a generating set ${\displaystyle g_{1},\ldots ,g_{k}}$ of a module M over a ring R, a relation or first syzygy between the generators is a k-tuple ${\displaystyle (a_{1},\ldots ,a_{k})}$ of elements of R such that[2]

${\displaystyle a_{1}g_{1}+\cdots +a_{k}g_{k}=0.}$

Let ${\displaystyle L_{0}}$ be the free module with basis ${\displaystyle (G_{1},\ldots ,G_{k}),}$ the relation ${\displaystyle (a_{1},\ldots ,a_{k})}$ may be identified with the element

${\displaystyle a_{1}G_{1}+\cdots +a_{k}G_{k},}$

and the relations form the kernel ${\displaystyle R_{1}}$ of the linear map ${\displaystyle L_{0}\to M}$ defined by ${\displaystyle G_{i}\mapsto g_{i}.}$ In other words, one has an exact sequence

${\displaystyle 0\to R_{1}\to L_{0}\to M\to 0.}$

This first syzygy module depends on the choice of a generating set, but, if ${\displaystyle S_{1}}$ is the module which is obtained with another generating set, there exist two free modules ${\displaystyle F_{1}}$ and ${\displaystyle F_{2}}$ such that

${\displaystyle R_{1}\oplus F_{1}\cong S_{1}\oplus F_{2}}$

where ${\displaystyle \oplus }$ denote the direct sum of modules.

The second syzygy module is the module of the relations between generators of the first syzygy module. By continuing in this way, one may define the kth syzygy module for every positive integer k.

If, for some k, the kth syzygy module is free, then, by taking a basis as a generating set, the next syzygy module (and every subsequent one) is the zero module. If one does not take a bases as generating sets, then all subsequent syzygy modules are free.

Let n be the smallest integer, if any, such that the nth syzygy module of a module M is free or projective. The above property of invariance, up to the sum direct with free modules, implies that n does not depend on the choice of generating sets. The projective dimension of M is this integer, if it exists, or if not. This is equivalent with the existence of an exact sequence

${\displaystyle 0\longrightarrow R_{n}\longrightarrow L_{n-1}\longrightarrow \cdots \longrightarrow L_{0}\longrightarrow M\longrightarrow 0,}$

where the modules ${\displaystyle L_{i}}$ are free and ${\displaystyle R_{n}}$ is projective. It can be shown that one may alway choose the generating sets for ${\displaystyle R_{n}}$ being free, that is for the above exact sequence to be a free resolution.

Statement

Hilbert's syzygy theorem states that, if M is a finitely generated module over a polynomial ring ${\displaystyle k[x_{1},\ldots ,x_{n}]}$ in n indeterminates over a field k, then the nth syzygy module of M is always a free module.

In modern language, this implies that the projective dimension of M is at most n, and thus that there exists a free resolution

${\displaystyle 0\longrightarrow L_{k}\longrightarrow L_{k-1}\longrightarrow \cdots \longrightarrow L_{0}\longrightarrow M\longrightarrow 0}$

of length kn.

This upper bound on the projective dimension is sharp, that is, there are modules of projective dimension exactly n. The standard example is the field k, which may be considered as a ${\displaystyle k[x_{1},\ldots ,x_{n}]}$-module by setting ${\displaystyle x_{i}c=0}$ for every i and every ck. For this module, the nth syzygy module is free, but not the (n − 1)th one (for a proof, see § Koszul complex, below).

The theorem is also true for modules that are not finitely generated. As the global dimension of a ring is the supremum of the projective dimensions of all modules, Hilbert's syzygy theorem may be restated as: the global dimension of ${\displaystyle k[x_{1},\ldots ,x_{n}]}$ is n.

Low dimension

In the case of zero indeterminates, Hilbert's syzygy theorem is simply the fact that every vector space has a basis.

In the case of a single indeterminate, Hilbert's syzygy theorem is an instance of the theorem asserting that over a principal ideal ring, every submodule of a free module is itself free.

Koszul complex

The Koszul complex, also called "complex of exterior algebra", allows, in some cases, an explicit description of all syzygy modules.

Let ${\displaystyle g_{1},\ldots ,g_{k}}$ be a generating system of an ideal I in a polynomial ring ${\displaystyle R=k[x_{1},\ldots ,x_{n}]}$, and let ${\displaystyle L_{1}}$ be a free module of basis ${\displaystyle G_{1},\ldots ,G_{k}.}$ The exterior algebra of ${\displaystyle L_{1}}$ is the direct sum

${\displaystyle \Lambda (L_{1})=\bigoplus _{t=0}^{k}L_{t},}$

where ${\displaystyle L_{t}}$ is the free module, which has, as a basis, the exterior products

${\displaystyle G_{i_{1}}\wedge \cdots \wedge G_{i_{t}},}$

such that ${\displaystyle i_{1} In particular, one has ${\displaystyle L_{0}=R}$ (because of the definition of the empty product), the two definitions of ${\displaystyle L_{1}}$ coincide, and ${\displaystyle L_{t}=0}$ for t > k. For every positive t, one may define a linear map ${\displaystyle L_{t}\to L_{t-1}}$ by

${\displaystyle G_{i_{1}}\wedge \cdots \wedge G_{i_{t}}\mapsto \sum _{j=1}^{t}(-1)^{j+1}g_{i_{j}}G_{i_{1}}\wedge \cdots \wedge {\widehat {G}}_{i_{j}}\wedge \cdots \wedge G_{i_{t}},}$

where the hat means that the factor is omitted. A straightforward computation shows that the composition of two consecutive such maps is zero, and thus that one has a complex

${\displaystyle 0\to L_{t}\to L_{t-1}\to \cdots \to L_{1}\to L_{0}\to R/I.}$

This is the Koszul complex. In general the Koszul complex is not an exact sequence, but it is an exact sequence if one works with a polynomial ring ${\displaystyle R=k[x_{1},\ldots ,x_{n}]}$ and an ideal generated by a regular sequence of homogeneous polynomials.

In particular, the sequence ${\displaystyle x_{1},\ldots ,x_{n}}$ is regular, and the Koszul complex is thus a projective resolution of ${\displaystyle k=R/\langle x_{1},\ldots ,x_{n}\rangle .}$ In this case, the nth syzygy module is free of dimension one (generated by the product of all ${\displaystyle G_{i}}$); the (n − 1)th syzygy module is thus the quotient of a free module of dimension n by the submodule generated by ${\displaystyle (x_{1},-x_{2},\ldots ,\pm x_{n}).}$ This quotient may not be a projective module, as otherwise, there would exist polynomials ${\displaystyle p_{i}}$ such that ${\displaystyle p_{1}x_{1}+\cdots +p_{n}x_{n}=1,}$ which is impossible (substituting the ${\displaystyle x_{i}}$ by 0 in the latter equality provides 1 = 0). This proves that the projective dimension of ${\displaystyle k=R/\langle x_{1},\ldots ,x_{n}\rangle }$ is exactly n.

The same proof applies for proving that the projective dimension of ${\displaystyle k[x_{1},\ldots ,x_{n}]/\langle g_{1},\ldots ,g_{t}\rangle }$ is exactly t if the ${\displaystyle g_{i}}$ form a regular sequence of homogeneous polynomials.

Computation

At Hilbert's time, there were no method available for computing syzygies. It was only known that an algorithm may be deduced from any upper bound of the degree of the generators of the module of syzygies. In fact, the coefficients of the syzygies are unknown polynomials. If the degree of these polynomials is bounded, the number of their monomials is also bounded. Expressing that one has a syzygy provides a system of linear equations whose unknowns are the coefficients of these monomials. Therefore, any algorithm for linear systems implies an algorithm for syzygies, as soon as a bound of the degrees is known.

The first bound for syzygies (as well as for ideal membership problem) was given in 1926 by Grete Hermann:[3] Let M a submodule of a free module L of dimension t over ${\displaystyle k[x_{1},\ldots ,x_{n}];}$ if the coefficients over a basis of L of a generating system of M have a total degree at most d, then there is a constant c such that the degrees occurring in of a generating system of the first syzygy module is at most ${\displaystyle (td)^{2^{cn}}.}$ The same bound applies for testing the membership to M of an element of L.[4]

On the other hand, there are examples where a double exponential degree necessarily occurs. However such examples are extremely rare, and this sets the question of an algorithm that is efficient when the output is not too large. At the present time, the best algorithms for computing syzygies are Gröbner basis algorithms. They allow the computation of the first syzygy module, and also, with almost no extra cost, all syzygies modules.

Syzygies and regularity

One might wonder which ring-theoretic property of ${\displaystyle A=k[x_{1},\ldots ,x_{n}]}$ causes the Hilbert syzygy theorem to hold. It turns out that this is regularity, which is an algebraic formulation of the fact that affine n-space is a variety without singularities. In fact the following generalization holds: Let ${\displaystyle A}$ be a Noetherian ring. Then ${\displaystyle A}$ has finite global dimension if and only if ${\displaystyle A}$ is regular and the Krull dimension of ${\displaystyle A}$ is finite; in that case the global dimension of ${\displaystyle A}$ is equal to the Krull dimension. This result may be proven using Serre's theorem on regular local rings.