When classifying objects in algebraic geometry, one usually fixes discrete
invariants, such as the genus of a projective curve, and then one would like
to have a distinct view on the set of objects with fixed invariants with
respect to some equivalence relation. For small invariants it is sometimes
possible to enumerate the equivalence classes and to provide normal forms.
For bigger invariants this usually fails and a way to describe the objects is
to construct a classifying space such that each point of this space
corresponds to a unique equivalence class. In algebraic geometry this
classifying space should again be an algebraic variety, together with certain
functorial properties. These ideas lead to the notion of a fine,
respectively coarse, **moduli space** ([MuF], [Ne]).

Classically, moduli spaces have been constructed for global algebraic objects such as projective varieties, or for vector bundles on a fixed projective variety. During the past years there has also been some progress in constructing moduli spaces for singularities (cf. [GHP]) and for Cohen-Macaulay modules on a fixed local ring of a curve singularity ([GP3], see also [Gr1] for a survey). Indeed, the methods of proof are constructible and can be transferred to algorithms and finally to programmes.

In the following, we describe an algorithm to compute a moduli space for isolated hypersurface singularities, following [GHP]. The algorithm has been developed and implemented in SINGULAR by T. Bayer ([Ba]).

Let
, , be a
weight vector
and
a semiquasihomogeneous power series, i.e.,

such that the quasi-homogeneous (or weighted homogeneous) principal part has an isolated singularity at the origin. We denote the class of all such power series (resp. singularities) by .

Two power series are called **right equivalent**,
, if there exists a holomorphic coordinate change
such that
,
or, equivalently, , where
is the algebra automorphism corresponding to .

In a series of papers, V.I. Arnold classified all isolated hypersurface singularities w.r.t. right equivalence up to modality 2, by giving normal forms [AGV].

Here we should like to present an algorithm to compute a moduli space for semiquasihomogeneous power series with fixed principal part w.r.t. right equivalence. In [GHP], also a moduli space for contact equivalence was constructed, but that construction is more involved and not treated here.

To start with we need an algebraic variety which parametrises all semiquasihomogeneous power series (up to right equivalence) and then to identify equivalent objects. Indeed, equivalent objects belong to the same orbit of an algebraic group action and the aim is to compute explicitly the group, the action of the group and, finally, the quotient space.

Giving , we compute the set of exponents so that is a monomial basis of the Milnor algebra . This requires a standard basis computation for a local ordering (cf. [GP1]). Then we select and set with .

The polynomial

is the miniversal -constant unfolding of . By a theorem of Arnold ([AGV]), for any there exists a such that . The next step in the algorithm is to compute, for a given , a coordinate change and a such that . The computation follows Arnold's proof, constructing degree by degree until the maximal weighted degree , .

Usually there exist such that . However, we have the following fact (proved in [GHP] by using the Gauß-Manin connection): let , and assume . Then , that is for .

In the theorem of Arnold,
, which implies that is unique. Moreover,
is a normal subgroup of
, and the quotient

acts algebraically on . Let denote the subgroup which fixes and denote by the image of . Then is a finite group acting algebraically on and the geometric quotient is the desired coarse moduli space for unfoldings in modulo right equivalence (cf. [GHP]).

The following steps are needed for computing the moduli space:

- 0.
- Compute miniversal -constant unfolding,
- 1.
- compute
,
- 2.
- compute the action of
on using
Arnold's theorem,
- 3.
- compute and linearise to get acting linearly on
some
, , and compute an equivariant embedding
,
- 4.
- determine generating invariant polynomials for ,
- 5.
- determine the relations between the invariants to get the equations for , which is the desired moduli space.

SINGULAR example for computing the moduli space (we omit intermediate commands):

> LIB "qhmoduli.lib"; > ring R = 0, (x,y,z), ls; // define a local ring > poly f = x2y + x2z + y5 - z5; // principal partStep 0: compute a basis for the semi-universal unfolding

B[1]=y3z3, B[2]=x2y3, B[3]=x2y2

Step 1, 2 and 3: compute the equations of the stabilizer of , compute the induced action on , linearise the action with equivariant embedding

> actionid; //linearised action of on

actionid[1]=s(1)*t(1), actionid[2]=-s(3)*t(2)+s(3)*t(4)+s(5)*t(2)

actionid[3]=s(4)*t(3), actionid[4]=s(5)*t(4)

> invars; //there are 21 invariants of degree 3 to 10

invars[1]=t(1)*t(4)^2, invars[2]=t(2)*t(3)*t(4)-t(3)*t(4)^2, ...

> setring R4; imageid; //simplified equation of moduli space

imageid[1]=Y(5)^2-Y(4)*Y(6), imageid[2]=Y(3)*Y(5)-Y(2)*Y(6), ...

imageid[55]=9*Y(1)^5+2816*Y(2)^2*Y(6)*Y(9)+296*Y(6)*Y(7)^2

-152*Y(2)*Y(6)*Y(8)-960*Y(6)*Y(7)*Y(9)-9*Y(6)*Y(10)

This shows already that moduli spaces have a complicated structure, even for relatively small examples.

**Problems:**

- Extend the algorithms to construct moduli spaces for singularities with
respect to contact equivalence. This will contain completely new parts
since we need not only handle finite groups but unipotent groups.
- Moduli spaces for torsion free modules on curve singularities have been constructed in [GP3] with constructive proofs. Again unipotent group actions come into play. It would be desirable to develop and implement algorithms and test conjectures related to the structure of these moduli spaces.