440 research outputs found
Classifying representations by way of Grassmannians
Let be a finite dimensional algebra over an algebraically closed
field. Criteria are given which characterize existence of a fine or coarse
moduli space classifying, up to isomorphism, the representations of
with fixed dimension and fixed squarefree top . Next to providing a
complete theoretical picture, some of these equivalent conditions are readily
checkable from quiver and relations. In case of existence of a moduli space --
unexpectedly frequent in light of the stringency of fine classification -- this
space is always projective and, in fact, arises as a closed subvariety
of a classical Grassmannian. Even when the full moduli
problem fails to be solvable, the variety is seen to
have distinctive properties recommending it as a substitute for a moduli space.
As an application, a characterization of the algebras having only finitely many
representations with fixed simple top is obtained; in this case of `finite
local representation type at a given simple ', the radical layering is shown to be a classifying invariant for the
modules with top . This relies on the following general fact obtained as a
byproduct: Proper degenerations of a local module never have the same
radical layering as
Finiteness results for Abelian tree models
Equivariant tree models are statistical models used in the reconstruction of
phylogenetic trees from genetic data. Here equivariant refers to a symmetry
group imposed on the root distribution and on the transition matrices in the
model. We prove that if that symmetry group is Abelian, then the Zariski
closures of these models are defined by polynomial equations of bounded degree,
independent of the tree. Moreover, we show that there exists a polynomial-time
membership test for that Zariski closure. This generalises earlier results on
tensors of bounded rank, which correspond to the case where the group is
trivial, and implies a qualitative variant of a quantitative conjecture by
Sturmfels and Sullivant in the case where the group and the alphabet coincide.
Our proofs exploit the symmetries of an infinite-dimensional projective limit
of Abelian star models.Comment: 27 pages. arXiv admin note: substantial text overlap with
arXiv:1103.533
On Parabolic Subgroups and Hecke Algebras of Some Fractal Groups
We study the subgroup structure, Hecke algebras, quasi-regular
representations, and asymptotic properties of some fractal groups of branch
type. We introduce parabolic subgroups, show that they are weakly maximal, and
that the corresponding quasi-regular representations are irreducible. These
(infinite-dimensional) representations are approximated by finite-dimensional
quasi-regular representations. The Hecke algebras associated to these parabolic
subgroups are commutative, so the decomposition in irreducible components of
the finite quasi-regular representations is given by the double cosets of the
parabolic subgroup. Since our results derive from considerations on
finite-index subgroups, they also hold for the profinite completions
of the groups G. The representations involved have interesting spectral
properties investigated in math.GR/9910102. This paper serves as a
group-theoretic counterpart to the studies in the mentionned paper. We study
more carefully a few examples of fractal groups, and in doing so exhibit the
first example of a torsion-free branch just-infinite group. We also produce a
new example of branch just-infinite group of intermediate growth, and provide
for it an L-type presentation by generators and relators.Comment: complement to math.GR/991010
- β¦