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## 3.3 The Hilbert series of resolutions

The Hilbert function has become an important tool for computations of Gröbner bases of quasihomogeneous ideals or modules during recent years. For a detailed discussion of the usage of Hilbert functions in that context I would like to refer to [GMRT].

Let the -module M be given by generators constituting a standard base of M with respect to the degree compatible ordering <.

Lemma 3..3   Resolving the initial module by the STZS-method, one obtains the Hilbert series as well as the numbers of generators and their degrees of the STZS-resolution of M.

Proof: Of course, this resolution of leading terms can be continued to a resolution of (see [MM]). The Hilbert functions are invariants of the initial modules (see [GMRT] Theorem 1) and from the properties of the STZS-resolution follows Syzi(In(M))=In(Syzi(M)).

Now, we assume to minimize the STZS-resolution Syzi(M), of M. This means, to find constant entres in the syzygies of level i and to cancel the corresponding element . The component il in Syzi(M) is set to zero by Gaussian eliminations and the syzygy is taken out together with its component in Syzi+1. (compare R. La Scala [LS] Chapter 4)

Denote by sfi-1 the set of superfluous generators in Syzi-1(M). By the descritpion above we get:

Lemma 3..4   The Hilbert function Hi of the i-th module in the minimal resolution is given by:

Remark: For computing the exact Hilbert function of the i-th module in the minimal resolution it is not necessary to know the concrete generators. It suffices to know the number and degree of a minimal set of generators. However, one has to be careful while counting explicit generators. They may be transformed by the Gaussian elimination or by cancellation of components of syzygies which are taken out into zero!

Theorem 1   By resolving the initial module (or ideal) of the input and detection of direct reductions in Syzi-1(M) one obtains the Hilbert function Hi of the module Syzi(M). The Hilbert functions HiSyzj for j>i are upper bounds for the Hj.

Proof: By searching for direct reductions one obtains a minimal set of generators for the actual module in the minimal resolution. Comparing them with the generators in the STZS-resolution one finds all superfluous elements in that resolution and can compute the Hilbert functions of the minimal resolution according to the lemmas above.

The interpretation of Hilbert functions in the context of syzygies differs from that in the Gröbner base algorithm. Once having detected all necessary pairs (and checked this by the Hilbert function) of a certain degree in the module Syzi-1(M), all remaining pairs of the same degree will yield syzygies. It seems to be a good idea to wait with their reduction until the next computation. Here one has a complete knowledge of the Hilbert function and can decide how many pairs must be reduced to obtain the correct Hilbert function in Syzi(M) up to the current degree. The rest of the pairs yields superfluous generators and their computation can be skipped. Therefore, the usage of Hilbert functions suggests a computation degree by degree over the whole resolvent.

Next: 3.4 The algorithm of Up: 3. Improvements Previous: 3.2 Direct minimizations
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