# Linear relation

In linear algebra, a **linear relation**, or simply **relation**, between elements of a vector space or a module is a linear equation that has these elements as a solution.

The construction of higher order syzygy modules is generalized as the definition of free resolutions, which allows restating Hilbert's syzygy theorem as *a polynomial ring in n indeterminates over a field has global homological dimension n.*

If a and b are two elements of the commutative ring R, then (*b*, –*a*) is a relation that is said *trivial*. The *module of trivial relations* of an ideal is the submodule of the first syzygy module of the ideal that is generated by the trivial relations between the elements of a generating set of an ideal. The concept of trivial relations can be generalized to higher order syzygy modules, and this leads to the concept of the Koszul complex of an ideal, which provides information on the non-trivial relations between the generators of an ideal.

If the ring R is Noetherian, or, at least coherent, and if M is finitely generated, then the syzygy module is also finitely generated. A syzygy module of this syzygy module is a *second syzygy module* of M. Continuing this way one can define a kth syzygy module for every positive integer k.

Generally speaking, in the language of K-theory, a property is *stable* if it becomes true by making a direct sum with a sufficiently large free module. A fundamental property of syzygies modules is that there are "stably independent" on choices of generating sets for involved modules. The following result is the basis of these stable properties.

For obtaining a similar result for higher syzygy modules, it remains to prove that, if M is any module, and L is a free module, then M and *M* ⊕ *L* have isomorphic syzygy modules. It suffices to consider a generating set of *M* ⊕ *L* that consists of a generating set of M and a basis of L. For every relation between the elements of this generating set, the coefficients of the basis elements of L are all zero, and the syzygies of *M* ⊕ *L* are exactly the syzygies of M extended with zero coefficients. This completes the proof to the following theorem.

One can repeat this construction with this kernel in place of M. Repeating again and again this construction, one gets a long exact sequence

The global dimension of a commutative Noetherian ring is either infinite, or the minimal n such that every free resolution is finite of length at most n. A commutative Noetherian ring is regular if its global dimension is finite. In this case, the global dimension equals its Krull dimension. So, Hilbert's syzygy theorem may be restated in a very short sentence that hides much mathematics: *A polynomial ring over a field is a regular ring.*

The word *syzygy* came into mathematics with the work of Arthur Cayley.^{[1]} In that paper, Cayley used it for in the theory of resultants and discriminants.^{[2]}
As the word syzygy was used in astronomy to denote a linear relation between planets, Cayley used it to denote linear relations between minors of a matrix, such as, in the case of a 2×3 matrix:

Then, the word *syzygy* was popularized (among mathematicians) by David Hilbert in his 1890 article, which contains three fundamental theorems on polynomials, Hilbert's syzygy theorem, Hilbert's basis theorem and Hilbert's Nullstellensatz.

In his article, Cayley makes use, in a special case, of what was later^{[3]} called the Koszul complex, after a similar construction in differential geometry by the mathematician Jean-Louis Koszul.