15,848 research outputs found
Comparison theorems for deformation functors via invariant theory
We compare deformations of algebras to deformations of schemes in the setting
of invariant theory. Our results generalize comparison theorems of Schlessinger
and the second author for projective schemes. We consider deformations
(abstract and embedded) of a scheme which is a good quotient of a
quasi-affine scheme by a linearly reductive group and compare
them to invariant deformations of an affine -scheme containing as
an open invariant subset. The main theorems give conditions for when the
comparison morphisms are smooth or isomorphisms.Comment: Minor improvements and corrections. Improved presentation. Changed
some definitions and term
Families of Artinian and one-dimensional algebras
The purpose of this paper is to study families of Artinian or one dimensional
quotients of a polynomial ring with a special look to level algebras. Let
\GradAlg^H(R) be the scheme parametrizing graded quotients of with
Hilbert function . Let be any graded surjection of quotients of
with Hilbert function and , and h-vectors
and , respectively. If \depth A = \dim A \leq
1 and is a ``truncation'' of in the sense that
for some , then we
show there is a close relationship between \GradAlg^{H_A}(R) and
\GradAlg^{H_B}(R) concerning e.g. smoothness and dimension at the points
and respectively, provided is a complete intersection or
provided the Castelnuovo-Mumford regularity of is at least 3 (sometimes 2)
larger than the regularity of . In the complete intersection case we
generalize this relationship to ``non-truncated'' Artinian algebras which
are compressed or close to being compressed. For more general Artinian algebras
we describe the dual of the tangent and obstruction space of deformations in a
manageable form which we make rather explicit for level algebras of
Cohen-Macaulay type 2. This description and a linkage theorem for families
allow us to prove a conjecture of Iarrobino on the existence of at least two
irreducible components of \GradAlg^H(R), , whose
general elements are Artinian level algebras of type 2.Comment: 29 page
Families of artinian and low dimensional determinantal rings
Let GradAlg(H) be the scheme parameterizing graded quotients of
R=k[x_0,...,x_n] with Hilbert function H (it is a subscheme of the Hilbert
scheme of P^n if we restrict to quotients of positive dimension, see definition
below). A graded quotient A=R/I of codimension c is called standard
determinantal if the ideal I can be generated by the t by t minors of a
homogeneous t by (t+c-1) matrix (f_{ij}). Given integers a_0\le a_1\le ...\le
a_{t+c-2} and b_1\le ...\le b_t, we denote by W_s(\underline{b};\underline{a})
the stratum of GradAlg(H) of determinantal rings where f_{ij} \in R are
homogeneous of degrees a_j-b_i.
In this paper we extend previous results on the dimension and codimension of
W_s(\underline{b};\underline{a}) in GradAlg(H) to {\it artinian determinantal
rings}, and we show that GradAlg(H) is generically smooth along
W_s(\underline{b};\underline{a}) under some assumptions. For zero and one
dimensional determinantal schemes we generalize earlier results on these
questions. As a consequence we get that the general element of a component W of
the Hilbert scheme of P^n is glicci provided W contains a standard
determinantal scheme satisfying some conditions. We also show how certain ghost
terms disappear under deformation while other ghost terms remain and are
present in the minimal resolution of a general element of GradAlg(H).Comment: Postprint replacing preprint. 29 pages. Online 26.May 2017 in Journal
of Pure and Applied Algebr
The Hilbert Scheme of Buchsbaum space curves
We consider the Hilbert scheme H(d,g) of space curves C with homogeneous
ideal I(C):=H_{*}^0(\sI_C) and Rao module M:=H_{*}^1(\sI_C). By taking suitable
generizations (deformations to a more general curve) C' of C, we simplify the
minimal free resolution of I(C) by e.g. making consecutive free summands
(ghost-terms) disappear in a free resolution of I(C'). Using this for Buchsbaum
curves of diameter one (M_v \ne 0 for only one v), we establish a one-to-one
correspondence between the set \sS of irreducible components of H(d,g) that
contain (C) and a set of minimal 5-tuples that specializes in an explicit
manner to a 5-tuple of certain graded Betti numbers of C related to
ghost-terms. Moreover we almost completely (resp. completely) determine the
graded Betti numbers of all generizations of C (resp. all generic curves of
\sS), and we give a specific description of the singular locus of the Hilbert
scheme of curves of diameter at most one. We also prove some semi-continuity
results for the graded Betti numbers of any space curve under some assumptions.Comment: Minor changes in Thm. 6.1 where the particular case (v) is corrected
(this inaccuracy occurs also in the published version in Annales de
l'institut Fourier, 2012); 23 page
Unobstructedness and dimension of families of Gorenstein algebras
The goal of this paper is to develop tools to study maximal families of
Gorenstein quotients A of a polynomial ring R. We prove a very general Theorem
on deformations of the homogeneous coordinate ring of a scheme Proj(A) which is
defined as the degeneracy locus of a regular section of the dual of some sheaf
M^~ of rank r supported on say an arithmetically Cohen-Macaulay subscheme
Proj(B) of Proj(R). Under certain conditions (notably; M maximally
Cohen-Macaulay and the top exterior power of M^~ a twist of the canonical
sheaf), then A is Gorenstein, and under additional assumptions, we show the
unobstructedness of A and we give an explicit formula the dimension of any
maximal family of Gorenstein quotients of R with fixed Hilbert function
obtained by a regular section as above. The theorem also applies to Artinian
quotients A.
The case where M itself is a twist of the canonical module (r=1) was studied
in a previous paper, while this paper concentrates on other low rank cases,
notably r=2 and 3. In these cases regular sections of the first Koszul homology
module and of normal sheaves to licci schemes (of say codimension 2) lead to
Gorenstein quotients (of e.g. codimension 4) whose parameter spaces we examine.
Our main applications are for Gorenstein quotients of codimension 4 of R since
our assumptions are almost always satisfied in this case. Special attention are
paid to arithmetically Gorenstein curves in P^5.Comment: 34 pages. In the 1st version on the arXiv, as well as in the
published version in Collect. Math. 58, 2 (2007), there is a missing
assumption of generality in the part of the results which deals with the
codimension of a stratum (see Rem. 16(ii) and the text before Thm. 15
The Hilbert Scheme of Space Curves of Small Diameter
This paper studies space curves C of degree d and arithmetic genus g, with
homogeneous ideal I and Rao module M = H_{*}^1(I^~), whose main results deal
with curves which satisfy Ext^2(M,M)_0=0 (e.g. of diameter, diam M < 3, which
means that M is non-vanishing in at most two consecutive degrees). For such
curves C we find necessary and sufficient conditions for unobstructedness, and
we compute the dimension of the Hilbert scheme, H(d,g), at (C) under the
sufficient conditions. In the diameter one case, the necessary and sufficient
conditions coincide, and the unobstructedness of C turns out to be equivalent
to the vanishing of certain products of graded Betti numbers of the free graded
minimal resolution of I. We give classes of obstructed curves C for which we
partially compute the equations of the singularity of H(d,g) at (C). Moreover
by taking suitable deformations we show how to kill certain repeated direct
free factors ("ghost-terms") in the minimal resolution of the ideal of the
general curve. For Buchsbaum curves of diameter at most 2, we simplify in this
way the minimal resolution further, allowing us to see when a singular point of
H(d,g) sits in the intersection of several, or lies in a unique irreducible
component of H(d,g). It follows that the products of the graded Betti numbers
mentioned above of a generic curve vanish, and that any irreducible component
of H(d,g) is reduced (generically smooth) in the diameter 1 case.Comment: Prop. 32(b) is now changed to what is stated in the published
version. In Prop. 32(a) we replace "minimal free resolution" by "free
resolution which is minimal except possibly in degree t+3", a change that
unfortunately is not done in the published version! A sentence in its proof
is added. There are changes just before Prop. 24 to credit Fl{\o}ystad, and
several changes on the "topos" leve
Families of low dimensional determinantal schemes
A scheme X \subset \PP^{n} of codimension c is called standard determinantal
if its homogeneous saturated ideal can be generated by the t x t minors of a
homogeneous t x (t+c-1) matrix (f_{ij}). Given integers a_0 \le a_1 \le ...\le
a_{t+c-2} and b_1 \le ...\le b_t, we denote by W_s(b;a) \subset Hilb(\PP^{n})
the stratum of standard determinantal schemes where f_{ij} are homogeneous
polynomials of degrees a_j-b_i and Hilb(\PP^{n}) is the Hilbert scheme (if n-c
> 0, resp. the postulation Hilbert scheme if n-c = 0).
Focusing mainly on zero and one dimensional determinantal schemes we
determine the codimension of W_s(b;a) in Hilb(\PP^{n}) and we show that
Hilb(\PP^{n}) is generically smooth along W_s(b;a) under certain conditions.
For zero dimensional schemes (only) we find a counterexample to the conjectured
value of dim W_s(b;a) appearing in [26].Comment: 22 page
On Fractional Instanton Numbers in Six Dimensional Heterotic E8 x E8 Orbifolds
We derive the precise relation between level matching condition and
fractional instanton numbers in six dimensional, abelian and supersymmetric
orbifolds of E8 x E8 heterotic string theory. The fractional part of the two E8
instanton numbers is explicitly calculated in terms of the gauge twist. This
relation is then used to show that the classification of these orbifolds can be
given in terms of flat bundles away from the orbifold singularities under the
only constraint that the sum of the fractional parts of the gauge instanton
numbers match the fractional part of the gravitational instanton number locally
at every fixed point. This directly carries over to M-theory on S^1/Z_2Comment: latex2e, 12 pages; reference and comments adde
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