981 research outputs found
Properties of the Fixed Point Lattice Dirac Operator in the Schwinger Model
We present a numerical study of the properties of the Fixed Point lattice
Dirac operator in the Schwinger model. We verify the theoretical bounds on the
spectrum, the existence of exact zero modes with definite chirality, and the
Index Theorem. We show by explicit computation that it is possible to find an
accurate approximation to the Fixed Point Dirac operator containing only very
local couplings.Comment: 38 pages, LaTeX, 3 figures, uses style [epsfig], a few comments and
relevant references adde
Controlled Sequential Monte Carlo
Sequential Monte Carlo methods, also known as particle methods, are a popular
set of techniques for approximating high-dimensional probability distributions
and their normalizing constants. These methods have found numerous applications
in statistics and related fields; e.g. for inference in non-linear non-Gaussian
state space models, and in complex static models. Like many Monte Carlo
sampling schemes, they rely on proposal distributions which crucially impact
their performance. We introduce here a class of controlled sequential Monte
Carlo algorithms, where the proposal distributions are determined by
approximating the solution to an associated optimal control problem using an
iterative scheme. This method builds upon a number of existing algorithms in
econometrics, physics, and statistics for inference in state space models, and
generalizes these methods so as to accommodate complex static models. We
provide a theoretical analysis concerning the fluctuation and stability of this
methodology that also provides insight into the properties of related
algorithms. We demonstrate significant gains over state-of-the-art methods at a
fixed computational complexity on a variety of applications
Change Acceleration and Detection
A novel sequential change detection problem is proposed, in which the change
should be not only detected but also accelerated. Specifically, it is assumed
that the sequentially collected observations are responses to treatments
selected in real time. The assigned treatments not only determine the
pre-change and post-change distributions of the responses, but also influence
when the change happens. The problem is to find a treatment assignment rule and
a stopping rule that minimize the expected total number of observations subject
to a user-specified bound on the false alarm probability. The optimal solution
to this problem is obtained under a general Markovian change-point model.
Moreover, an alternative procedure is proposed, whose applicability is not
restricted to Markovian change-point models and whose design requires minimal
computation. For a large class of change-point models, the proposed procedure
is shown to achieve the optimal performance in an asymptotic sense. Finally,
its performance is found in two simulation studies to be close to the optimal,
uniformly with respect to the error probability
Fractal Structure and Generalization Properties of Stochastic Optimization Algorithms
Understanding generalization in deep learning has been one of the major
challenges in statistical learning theory over the last decade. While recent
work has illustrated that the dataset and the training algorithm must be taken
into account in order to obtain meaningful generalization bounds, it is still
theoretically not clear which properties of the data and the algorithm
determine the generalization performance. In this study, we approach this
problem from a dynamical systems theory perspective and represent stochastic
optimization algorithms as random iterated function systems (IFS). Well studied
in the dynamical systems literature, under mild assumptions, such IFSs can be
shown to be ergodic with an invariant measure that is often supported on sets
with a fractal structure. As our main contribution, we prove that the
generalization error of a stochastic optimization algorithm can be bounded
based on the `complexity' of the fractal structure that underlies its invariant
measure. Leveraging results from dynamical systems theory, we show that the
generalization error can be explicitly linked to the choice of the algorithm
(e.g., stochastic gradient descent -- SGD), algorithm hyperparameters (e.g.,
step-size, batch-size), and the geometry of the problem (e.g., Hessian of the
loss). We further specialize our results to specific problems (e.g.,
linear/logistic regression, one hidden-layered neural networks) and algorithms
(e.g., SGD and preconditioned variants), and obtain analytical estimates for
our bound.For modern neural networks, we develop an efficient algorithm to
compute the developed bound and support our theory with various experiments on
neural networks.Comment: 34 pages including Supplement, 4 Figure
Fast space-variant elliptical filtering using box splines
The efficient realization of linear space-variant (non-convolution) filters
is a challenging computational problem in image processing. In this paper, we
demonstrate that it is possible to filter an image with a Gaussian-like
elliptic window of varying size, elongation and orientation using a fixed
number of computations per pixel. The associated algorithm, which is based on a
family of smooth compactly supported piecewise polynomials, the
radially-uniform box splines, is realized using pre-integration and local
finite-differences. The radially-uniform box splines are constructed through
the repeated convolution of a fixed number of box distributions, which have
been suitably scaled and distributed radially in an uniform fashion. The
attractive features of these box splines are their asymptotic behavior, their
simple covariance structure, and their quasi-separability. They converge to
Gaussians with the increase of their order, and are used to approximate
anisotropic Gaussians of varying covariance simply by controlling the scales of
the constituent box distributions. Based on the second feature, we develop a
technique for continuously controlling the size, elongation and orientation of
these Gaussian-like functions. Finally, the quasi-separable structure, along
with a certain scaling property of box distributions, is used to efficiently
realize the associated space-variant elliptical filtering, which requires O(1)
computations per pixel irrespective of the shape and size of the filter.Comment: 12 figures; IEEE Transactions on Image Processing, vol. 19, 201
Temporal breakdown and Borel resummation in the complex Langevin method
We reexamine the Parisi-Klauder conjecture for complex e^{i\theta/2} \phi^4
measures with a Wick rotation angle 0 <= \theta/2 < \pi/2 interpolating between
Euclidean and Lorentzian signature. Our main result is that the asymptotics for
short stochastic times t encapsulates information also about the equilibrium
aspects. The moments evaluated with the complex measure and with the real
measure defined by the stochastic Langevin equation have the same t -> 0
asymptotic expansion which is shown to be Borel summable. The Borel transform
correctly reproduces the time dependent moments of the complex measure for all
t, including their t -> infinity equilibrium values. On the other hand the
results of a direct numerical simulation of the Langevin moments are found to
disagree from the `correct' result for t larger than a finite t_c. The
breakdown time t_c increases powerlike for decreasing strength of the noise's
imaginary part but cannot be excluded to be finite for purely real noise. To
ascertain the discrepancy we also compute the real equilibrium distribution for
complex noise explicitly and verify that its moments differ from those obtained
with the complex measure.Comment: title changed, results on parameter dependence of t_c added,
exposition improved. 39 pages, 7 figure
A recursive-faulting model of distributed damage in confined brittle materials
We develop a model of distributed damage in brittle materials deforming in triaxial compression based on the explicit construction of special microstructures obtained by recursive faulting. The model aims to predict the effective or macroscopic behavior of the material from its elastic and fracture properties; and to predict the microstructures underlying the microscopic behavior. The model accounts for the elasticity of the matrix, fault nucleation and the cohesive and frictional behavior of the faults. We analyze the resulting quasistatic boundary value problem and determine the relaxation of the potential energy, which describes the macroscopic material behavior averaged over all possible fine-scale structures. Finally, we present numerical calculations of the dynamic multi-axial compression experiments on sintered aluminum nitride of Chen and Ravichandran [1994. Dynamic compressive behavior of ceramics under lateral confinement. J. Phys. IV 4, 177–182; 1996a. Static and dynamic compressive behavior of aluminum nitride under moderate confinement. J. Am. Soc. Ceramics 79(3), 579–584; 1996b. An experimental technique for imposing dynamic multiaxial compression with mechanical confinement. Exp. Mech. 36(2), 155–158; 2000. Failure mode transition in ceramics under dynamic multiaxial compression. Int. J. Fracture 101, 141–159]. The model correctly predicts the general trends regarding the observed damage patterns; and the brittle-to-ductile transition resulting under increasing confinement
Euler flag enumeration of Whitney stratified spaces
The flag vector contains all the face incidence data of a polytope, and in
the poset setting, the chain enumerative data. It is a classical result due to
Bayer and Klapper that for face lattices of polytopes, and more generally,
Eulerian graded posets, the flag vector can be written as a cd-index, a
non-commutative polynomial which removes all the linear redundancies among the
flag vector entries. This result holds for regular CW complexes.
We relax the regularity condition to show the cd-index exists for Whitney
stratified manifolds by extending the notion of a graded poset to that of a
quasi-graded poset. This is a poset endowed with an order-preserving rank
function and a weighted zeta function. This allows us to generalize the
classical notion of Eulerianness, and obtain a cd-index in the quasi-graded
poset arena. We also extend the semi-suspension operation to that of embedding
a complex in the boundary of a higher dimensional ball and study the simplicial
shelling components.Comment: 41 pages, 3 figures. Final versio
- …