4,151 research outputs found
Estimate of the number of one-parameter families of modules over a tame algebra
The problem of classifying modules over a tame algebra A reduces to a block
matrix problem of tame type whose indecomposable canonical matrices are zero-
or one-parameter. Respectively, the set of nonisomorphic indecomposable modules
of dimension at most d divides into a finite number f(d,A) of modules and
one-parameter series of modules.
We prove that the number of m-by-n canonical parametric block matrices with a
given partition into blocks is bounded by 4^s, where s is the number of free
entries (which is at most mn), and estimate the number f(d,A).Comment: 23 page
Estimate of the number of one-parameter families of modules over a tame algebra
The problem of classifying modules over a tame algebra A reduces to a block
matrix problem of tame type whose indecomposable canonical matrices are zero-
or one-parameter. Respectively, the set of nonisomorphic indecomposable modules
of dimension at most d divides into a finite number f(d,A) of modules and
one-parameter series of modules.
We prove that the number of m-by-n canonical parametric block matrices with a
given partition into blocks is bounded by 4^s, where s is the number of free
entries (which is at most mn), and estimate the number f(d,A).Comment: 23 page
Quantifier elimination in C*-algebras
The only C*-algebras that admit elimination of quantifiers in continuous
logic are , Cantor space and
. We also prove that the theory of C*-algebras does not have
model companion and show that the theory of is not
-axiomatizable for any .Comment: More improvements and bug fixes. To appear in IMR
Covariance and Time Regained in Canonical General Relativity
Canonical vacuum gravity is expressed in generally-covariant form in order
that spacetime diffeomorphisms be represented within its equal-time phase
space. In accordance with the principle of general covariance, the time mapping
{\T}: {\yman} \to {\rman} and the space mapping {\X}: {\yman} \to {\xman}
that define the Dirac-ADM foliation are incorporated into the framework of the
Hilbert variational principle. The resulting canonical action encompasses all
individual Dirac-ADM actions, corresponding to different choices of foliating
vacuum spacetimes by spacelike hypersurfaces. In this framework, spacetime
observables, namely, dynamical variables that are invariant under spacetime
diffeomorphisms, are not necessarily invariant under the deformations of the
mappings \T and \X, nor are they constants of the motion. Dirac observables
form only a subset of spacetime observables that are invariant under the
transformations of \T and \X and do not evolve in time. The conventional
interpretation of the canonical theory, due to Bergmann and Dirac, can be
recovered only by postulating that the transformations of the reference system
({\T},{\X}) have no measurable consequences. If this postulate is not deemed
necessary, covariant canonical gravity admits no classical problem of time.Comment: 41 pages, no figure
Peculiarities of massive vectormesons and their zero mass limits
Massive QED, in contrast with its massless counterpart, possesses two
conserved charges; one is a screened (vanishing) Maxwell charge which is
directly associated with the massive vector mesons through the identically
conserved Maxwell current. A somewhat peculiar situation arises for couplings
of Hermitian matter fields to massive vector potentials; in that case the only
current is the screened Maxwell current and the coupling disappears in the
massless limit. In case of selfinteracting massive vector mesons the situation
becomes even more peculiar in that the usually renormalizability guaranteeing
validity of the first order power-counting criterion breaks down in second
order and requires the compensatory presence of additional Hermitian H-fields.
Some aspect of these observation have already been noticed in the BRST gauge
theoretic formulation, but here we use a new setting based on string-local
vector mesons which is required by Hilbert space positivity. The coupling to
H-fields induces Mexican hat like selfinteractions; they are not imposed and
bear no relation with spontaneous symmetry breaking; they are rather
consequences of the foundational causal localization properties realized in a
Hilbert space setting. In case of selfinteracting massive vectormesons their
presence is required in order to maintain the first order power-counting
restriction of renormalizability also in second order. The presentation of the
new Hilbert space setting for vector mesons which replaces gauge theory and
extends on-shell unitarity to its off-shell counterpart is the main motivation
for this work. The new Hilbert space setting also shows that the second order
Lie-algebra structure of selfinteracting vector mesons is a consequence of the
principles of QFT and promises a deeper understanding of the origin of
confinement.Comment: 34 pages Latex, several additional remarks and citations, improved
formulations, same as published versio
The Structure of GUT Breaking by Orbifolding
Recently, an attractive model of GUT breaking has been proposed in which a 5
dimensional supersymmetric SU(5) gauge theory on an S^1/(Z_2\times Z_2')
orbifold is broken down to the 4d MSSM by SU(5)-violating boundary conditions.
Motivated by this construction and several related realistic models, we
investigate the general structure of orbifolds in the effective field theory
context, and of this orbifold symmetry breaking mechanism in particular. An
analysis of the group theoretic structure of orbifold breaking is performed.
This depends upon the existence of appropriate inner and outer automorphisms of
the Lie algebra, and we show that a reduction of the rank of the GUT group is
possible. Some aspects of larger GUT theories based on SO(10) and E_6 are
discussed. We explore the possibilities of defining the theory directly on a
space with boundaries and breaking the gauge symmetry by more general
consistently chosen boundary conditions for the fields. Furthermore, we derive
the relation of orbifold breaking with the familiar mechanism of Wilson line
breaking, finding a one-to-one correspondence, both conceptually and
technically. Finally, we analyse the consistency of orbifold models in the
effective field theory context, emphasizing the necessity for self-adjoint
extensions of the Hamiltonian and other conserved operators, and especially the
highly restrictive anomaly cancellation conditions that apply if the bulk
theory lives in more than 5 dimensions.Comment: Latex, 23 pages, version to be published in Nucl.Phys.
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