1,205 research outputs found
Reconstructing projective modules from its trace ideal
We make a detailed study of idempotent ideals that are traces of countably
generated projective right modules. We associate to such ideals an ascending
chain of finitely generated left ideals and, dually, a descending chain of
cofinitely generated right ideals.
The study of the first sequence allows us to characterize trace ideals of
projective modules and to show that projective modules can always be lifted
modulo the trace ideal of a projective module. As a consequence we give some
new classification results of (countably generated) projective modules over
particular classes of semilocal rings. The study of the second sequence leads
us to consider projective modules over noetherian FCR-algebras; we make some
constructions of non-trivial projective modules showing that over such rings
the behavior of countably generated projective modules that are not direct sum
of finitely generated ones is, in general, quite complex.Comment: 29 page
Automata and rational expressions
This text is an extended version of the chapter 'Automata and rational
expressions' in the AutoMathA Handbook that will appear soon, published by the
European Science Foundation and edited by JeanEricPin
Lifting defects for nonstable K_0-theory of exchange rings and C*-algebras
The assignment (nonstable K_0-theory), that to a ring R associates the monoid
V(R) of Murray-von Neumann equivalence classes of idempotent infinite matrices
with only finitely nonzero entries over R, extends naturally to a functor. We
prove the following lifting properties of that functor: (1) There is no functor
F, from simplicial monoids with order-unit with normalized positive
homomorphisms to exchange rings, such that VF is equivalent to the identity.
(2) There is no functor F, from simplicial monoids with order-unit with
normalized positive embeddings to C*-algebras of real rank 0 (resp., von
Neumann regular rings), such that VF is equivalent to the identity. (3) There
is a {0,1}^3-indexed commutative diagram D of simplicial monoids that can be
lifted, with respect to the functor V, by exchange rings and by C*-algebras of
real rank 1, but not by semiprimitive exchange rings, thus neither by regular
rings nor by C*-algebras of real rank 0. By using categorical tools from an
earlier paper (larders, lifters, CLL), we deduce that there exists a unital
exchange ring of cardinality aleph three (resp., an aleph three-separable
unital C*-algebra of real rank 1) R, with stable rank 1 and index of nilpotence
2, such that V(R) is the positive cone of a dimension group and V(R) is not
isomorphic to V(B) for any ring B which is either a C*-algebra of real rank 0
or a regular ring.Comment: 34 pages. Algebras and Representation Theory, to appea
Noetherianity for infinite-dimensional toric varieties
We consider a large class of monomial maps respecting an action of the
infinite symmetric group, and prove that the toric ideals arising as their
kernels are finitely generated up to symmetry. Our class includes many
important examples where Noetherianity was recently proved or conjectured. In
particular, our results imply Hillar-Sullivant's Independent Set Theorem and
settle several finiteness conjectures due to Aschenbrenner, Martin del Campo,
Hillar, and Sullivant.
We introduce a matching monoid and show that its monoid ring is Noetherian up
to symmetry. Our approach is then to factorize a more general equivariant
monomial map into two parts going through this monoid. The kernels of both
parts are finitely generated up to symmetry: recent work by
Yamaguchi-Ogawa-Takemura on the (generalized) Birkhoff model provides an
explicit degree bound for the kernel of the first part, while for the second
part the finiteness follows from the Noetherianity of the matching monoid ring.Comment: 20 page
Finite convergent presentations of plactic monoids for semisimple lie algebras
We study rewriting properties of the column presentation of plactic monoid
for any semisimple Lie algebra such as termination and confluence. Littelmann
described this presentation using L-S paths generators. Thanks to the shapes of
tableaux, we show that this presentation is finite and convergent. We obtain as
a corollary that plactic monoids for any semisimple Lie algebra satisfy
homological finiteness properties
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