9,598 research outputs found
Simulation of a model-based optimal controller for heating systems under realistic hypothesis
An optimal controller for auxiliary heating of passive solar buildings and commercial buildings with high internal gains is tested in simulation. Some of the most restrictive simplifications that were used in previous studies of that controller (Kummert et al., 2001) are lifted: the controller is applied to a multizone building, and a detailed model is used for the HVAC system. The model-based control algorithm is not modified. It is based on a simplified internal model
Unique Continuation for the Magnetic Schr\"odinger Equation
The unique-continuation property from sets of positive measure is here proven
for the many-body magnetic Schr\"odinger equation. This property guarantees
that if a solution of the Schr\"odinger equation vanishes on a set of positive
measure, then it is identically zero. We explicitly consider potentials written
as sums of either one-body or two-body functions, typical for Hamiltonians in
many-body quantum mechanics. As a special case, we are able to treat atomic and
molecular Hamiltonians. The unique-continuation property plays an important
role in density-functional theories, which underpins its relevance in quantum
chemistry
Coulomb gauge studies of SU(3) Yang-Mills theory on the lattice
We study the infrared behaviour of lattice SU(3) Yang-Mills theory in Coulomb
gauge in terms of the ghost propagator, the Coulomb potential and the
transversal and the time-time component of the equal-time gluon propagator. In
particular, we focus on the Gribov problem and its impact on the observables.
We observe that the simulated annealing method is advantageous for fixing the
Coulomb gauge in large volumes. We study finite size and discretization
effects. While finite size effects can be controlled by the cone cut, and the
ghost propagator and the Coulomb potential become scaling functions with the
cylinder cut, the equal-time gluon propagator does not show scaling in the
considered range of the inverse coupling constant. The ghost propagator is
infrared enhanced. The Coulomb potential is now extended to considerably lower
momenta and shows a more complicated infrared regime. The Coulomb string
tension satisfies Zwanziger's inequality, but its estimate can be considered
only preliminary because of the systematic Gribov effect that is particularly
strong for the Coulomb potential.Comment: 7 pages, 5 pictures, poster presented at the XXV International
Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg,
Germany; corrected value for fitting parameter
Perturbative corrections to the determination of Vub from the P+ spectrum in B->X_u l nu
We investigate the relation between the E_gamma spectrum in B->X_s gamma
decay and the P+ spectrum in semileptonic B->X_u l nu decay (P+ is the hadronic
energy minus the absolute value of the hadronic three-momentum), which provides
in principle the theoretically simplest determination of Vub from any of the
"shape function regions" of B->X_u l nu spectra. We calculate analytically the
P+ spectrum to order alpha_s^2 beta_0, and study its relation to the B->X_s
gamma photon spectrum to eliminate the leading dependence on nonperturbative
effects. We compare the result of fixed order perturbation theory to the
next-to-leading log renormalization group improved calculation, and argue that
fixed order perturbation theory is likely to be a more appropriate expansion.
Implications for the perturbative uncertainties in the determination of Vub
from the P+ spectrum are discussed.Comment: reference added, to appear in PR
Hierarchical quantum master equation approach to charge transport in molecular junctions with time-dependent molecule-lead coupling strengths
Time-dependent currents in molecular junctions can be caused by structural
fluctuations or interaction with external fields. In this publication, we
demonstrate how the hierarchical quantum master equation approach can be used
to study time-dependent transport in a molecular junction. This reduced density
matrix methodology provides a numerically exact solution to the transport
problem including time-dependent energy levels, molecule-lead coupling
strengths and transitions between electronic states of the molecular bridge.
Based on a representative model, the influence of a time-dependent
molecule-lead coupling on the electronic current is analyzed in some detail
A Variational Perspective on Accelerated Methods in Optimization
Accelerated gradient methods play a central role in optimization, achieving
optimal rates in many settings. While many generalizations and extensions of
Nesterov's original acceleration method have been proposed, it is not yet clear
what is the natural scope of the acceleration concept. In this paper, we study
accelerated methods from a continuous-time perspective. We show that there is a
Lagrangian functional that we call the \emph{Bregman Lagrangian} which
generates a large class of accelerated methods in continuous time, including
(but not limited to) accelerated gradient descent, its non-Euclidean extension,
and accelerated higher-order gradient methods. We show that the continuous-time
limit of all of these methods correspond to traveling the same curve in
spacetime at different speeds. From this perspective, Nesterov's technique and
many of its generalizations can be viewed as a systematic way to go from the
continuous-time curves generated by the Bregman Lagrangian to a family of
discrete-time accelerated algorithms.Comment: 38 pages. Subsumes an earlier working draft arXiv:1509.0361
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