12,569 research outputs found
Query Complexity of Derivative-Free Optimization
This paper provides lower bounds on the convergence rate of Derivative Free
Optimization (DFO) with noisy function evaluations, exposing a fundamental and
unavoidable gap between the performance of algorithms with access to gradients
and those with access to only function evaluations. However, there are
situations in which DFO is unavoidable, and for such situations we propose a
new DFO algorithm that is proved to be near optimal for the class of strongly
convex objective functions. A distinctive feature of the algorithm is that it
uses only Boolean-valued function comparisons, rather than function
evaluations. This makes the algorithm useful in an even wider range of
applications, such as optimization based on paired comparisons from human
subjects, for example. We also show that regardless of whether DFO is based on
noisy function evaluations or Boolean-valued function comparisons, the
convergence rate is the same
Active Learning for Undirected Graphical Model Selection
This paper studies graphical model selection, i.e., the problem of estimating
a graph of statistical relationships among a collection of random variables.
Conventional graphical model selection algorithms are passive, i.e., they
require all the measurements to have been collected before processing begins.
We propose an active learning algorithm that uses junction tree representations
to adapt future measurements based on the information gathered from prior
measurements. We prove that, under certain conditions, our active learning
algorithm requires fewer scalar measurements than any passive algorithm to
reliably estimate a graph. A range of numerical results validate our theory and
demonstrates the benefits of active learning.Comment: AISTATS 201
Laser induced magnetization switching in films with perpendicular anisotropy: a comparison between measurements and a multi-macrospin model
Thermally-assisted ultra-fast magnetization reversal in a DC magnetic field
for magnetic multilayer thin films with perpendicular anisotropy has been
investigated in the time domain using femtosecond laser heating. The experiment
is set-up as an optically pumped stroboscopic Time Resolved Magneto-Optical
Kerr Effect magnetometer. It is observed that a modest laser fluence of about
0.3 mJ/square-cm induces switching of the magnetization in an applied field
much less than the DC coercivity (0.8 T) on the sub-nanosecond time-scale. This
switching was thermally-assisted by the energy from the femtosecond pump-pulse.
The experimental results are compared with a model based on the Landau
Lifschitz Bloch equation. The comparison supports a description of the reversal
process as an ultra-fast demagnetization and partial recovery followed by
slower thermally activated switching due to the spin system remaining at an
elevated temperature after the heating pulse.Comment: 8 pages, 10 figures, to be submitted to PR
Entropically driven transition to a liquid-crystalline polymer globule
A self-consistent-field theory (SCFT) in the grand canonical ensemble
formulation is used to study transitions in a helix-coil multiblock copolymer
globule. The helices are modeled as stiff rods. In addition to the established
coil-globule transition we show for the first time that, even without explicit
rod-rod alignment interaction, the system undergoes a transition to a nematic
liquid-crystalline (LC) globular state. The LC-globule formation is driven by
the hydrophobic helical segment attraction and the anisotropy of the globule
surface energy. The full phase diagram of the copolymer was calculated. It
discriminates between an open chain, amorphous globule and LC-globule. This
model provides a relatively simple example of the interplay between secondary
and tertiary structures in homopolypeptides. Moreover, it gives a simple
explanation for the formation of helix bundles in certain globular proteins.Comment: 5 pages, 5 figures, submitted to Europhys. Let
Influence of the temperature on the depinning transition of driven interfaces
We study the dynamics of a driven interface in a two-dimensional random-field
Ising model close to the depinning transition at small but finite temperatures
T using Glauber dynamics. A square lattice is considered with an interface
initially in (11)-direction. The drift velocity v is analyzed for the first
time using finite size scaling at T = 0 and additionally finite temperature
scaling close to the depinning transition. In both cases a perfect data
collapse is obtained from which we deduce beta = 1/3 for the exponent which
determines the dependence of v on the driving field, nu = 1 for the exponent of
the correlation length and delta = 5 for the exponent which determines the
dependence of v on T.Comment: 5 pages, Latex, Figures included, to appear in Europhys. Let
Active Ranking using Pairwise Comparisons
This paper examines the problem of ranking a collection of objects using
pairwise comparisons (rankings of two objects). In general, the ranking of
objects can be identified by standard sorting methods using
pairwise comparisons. We are interested in natural situations in which
relationships among the objects may allow for ranking using far fewer pairwise
comparisons. Specifically, we assume that the objects can be embedded into a
-dimensional Euclidean space and that the rankings reflect their relative
distances from a common reference point in . We show that under this
assumption the number of possible rankings grows like and demonstrate
an algorithm that can identify a randomly selected ranking using just slightly
more than adaptively selected pairwise comparisons, on average. If
instead the comparisons are chosen at random, then almost all pairwise
comparisons must be made in order to identify any ranking. In addition, we
propose a robust, error-tolerant algorithm that only requires that the pairwise
comparisons are probably correct. Experimental studies with synthetic and real
datasets support the conclusions of our theoretical analysis.Comment: 17 pages, an extended version of our NIPS 2011 paper. The new version
revises the argument of the robust section and slightly modifies the result
there to give it more impac
Antiarrhythmic effect of the calcium antagonist tiapamil (ro 11–1781) by intravenous administration in patients with coronary heart disease
Twenty coronary patients with a median age of 76 years were treated in the coronary care unit with tiapamil, a new Ca2+ antagonist, by intravenous infusion (until December, 1979, the generic name was dimeditiapramine). The following arrhythmias were identified: atrial fibrillation with ventricular rate greater than 95 beats/min (5 patients); supraventricular premature complexes (SVPC) (4 patients); and ventricular premature complexes (VPC), Lown grades 2-4 (15 patients). Electrocardiograms and hemodynamic parameters were continuously monitored prior to, during, and after the therapy. In patients with atrial fibrillation, sinus rhythm was not restored, but tiapamil decreased the ventricular rate by 54%. In patients with VPC, the median frequency of VPC decreased from 310.5 before tiapamil to 32.5 beats/h at the fourth hour of therapy (p less than 0.01). The median ectopic/sinus beat ratio decreased from 0.083 (pretreatment) to 0.008 at the fourth hour of infusion (p less than 0.10). In one of the patient with an insufficient decrease in the number of VPC, the VPOC changed from class 4a (pretreatment) to class 2 (during the therapy), returning to class 4a after the infusion was stopped. Tiapamil reduced the median systolic and diastolic blood pressures by 8.3 and 7.1%, respectively (p less than 0.05), the third hour. Hypotension and bradycardia were observed in 5/20 patients. The results show that tiapamil is effective against both supraventricular and ventricular arrhythmias, and thus its spectrum of action differs from that of other calcium antagonists
Percolation, depinning, and avalanches in capillary condensation of gases in disordered porous solids
We propose a comprehensive theoretical description of hysteresis in capillary
condensation of gases in mesoporous disordered materials. Applying mean-field
density functional theory to a coarse-grained lattice-gas model, we show that
the morphology of the hysteresis loops is influenced by out-of-equilibrium
transitions that are different on filling and on draining. In particular,
desorption may be associated to a depinning process and be percolation-like
without explicit pore-blocking effects.Comment: 4 pages, 5 figure
Half-Skyrmions, Tensor Forces and Symmetry Energy in Cold Dense Matter
In a previous article, the 4D half-skyrmion (or 5D dyonic salt) structure of
dense baryonic matter described in crystalline configuration in the large
limit was shown to impact nontrivially on how anti-kaons behave in compressed
nuclear matter with a possible implication on an "ice-9" phenomenon of deeply
bound kaonic matter and condensed kaons in compact stars. We extend the
analysis to make a further prediction on the scaling properties of hadrons that
have a surprising effect on the nuclear tensor forces, the symmetry energy and
hence on the phase structure at high density. We treat this problem relying on
certain topological structure of chiral solitons. Combined with what can be
deduced from hidden local symmetry for hadrons in dense medium and the "soft"
dilatonic degree of freedom associated with the trace anomaly of QCD, we
uncover a novel structure of chiral symmetry in the "supersoft" symmetry energy
that can influence the structure of neutron stars.Comment: 8 pages, 4 figures; contents unchanged but expanded for a journa
Disordered Environments in Spatial Games
The Prisoner's dilemma is the main game theoretical framework in which the
onset and maintainance of cooperation in biological populations is studied. In
the spatial version of the model, we study the robustness of cooperation in
heterogeneous ecosystems in spatial evolutionary games by considering site
diluted lattices. The main result is that due to disorder, the fraction of
cooperators in the population is enhanced. Moreover, the system presents a
dynamical transition at , separating a region with spatial chaos from
one with localized, stable groups of cooperators.Comment: 6 pages, 5 figure
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