47,769 research outputs found
Asymptotic Analysis of Machine Learning Models: Comparison Theorems and Universality
The study of Machine Learning models in asymptotic regimes, has provided insight into many of the properties of ML models, but seemingly contradicts classical statistical wisdom. To solve this mystery, this thesis focuses on the analysis of models such as the LASSO and Random features regression, when the data points and model parameters grow infinite at constant ratios. It provides analysis for the asymptotic behavior of these problems, including characterization of the learning curves; the predicted training and generalization error as a function of the degree of overparameterization. The papers in this thesis particularly focus on the usage of Gaussian comparison theorems as a methodological tool for the analysis of these problems. In particular, the convex Gaussian min max theorem allows us to study more complex ML optimization problems, by considering alternative models that are simpler to analyze, but asymptotically hold similar properties. Secondarily, this thesis considers universality, which within the asymptotic context demonstrates that many statistics of ML models are fully determined by lower order statistical moments. This allows us to study surrogate Gaussian models, matching these moments. These surrogate Gaussian models can subsequently be analyzed by means of the Gaussian comparison theorems
On the Distribution of Quadratic Expressions in Various Types of Random Vectors
Several approximations to the distribution of indefinite quadratic expressions in possibly singular Gaussian random vectors and ratios thereof are obtained in this dissertation. It is established that such quadratic expressions can be represented in their most general form as the difference of two positive definite quadratic forms plus a linear combination of Gaussian random variables. New advances on the distribution of quadratic expressions in elliptically contoured vectors, which are expressed as scalar mixtures of Gaussian vectors, are proposed as well. Certain distributional aspects of Hermitian quadratic expressions in complex Gaussian vectors are also investigated. Additionally, approximations to the distributions of quadratic forms in uniform, beta, exponential and gamma random variables as well as order statistics thereof are determined from their exact moments, for which explicit representations are derived. Closed form representations of the approximations to the density functions of the various types of quadratic expressions being considered herein are obtained by adjusting the base density functions associated with the quadratic forms appearing in the decompositions of the expressions by means of polynomials whose coefficients are determined from the moments of the target distributions. Quadratic forms being ubiquitous in Statistics, the proposed distributional results should prove eminently useful
On statistics of bi-orthogonal eigenvectors in real and complex Ginibre ensembles: combining partial Schur decomposition with supersymmetry
We suggest a method of studying the joint probability density (JPD) of an
eigenvalue and the associated 'non-orthogonality overlap factor' (also known as
the 'eigenvalue condition number') of the left and right eigenvectors for
non-selfadjoint Gaussian random matrices of size . First we derive
the general finite expression for the JPD of a real eigenvalue
and the associated non-orthogonality factor in the real Ginibre ensemble, and
then analyze its 'bulk' and 'edge' scaling limits. The ensuing distribution is
maximally heavy-tailed, so that all integer moments beyond normalization are
divergent. A similar calculation for a complex eigenvalue and the
associated non-orthogonality factor in the complex Ginibre ensemble is
presented as well and yields a distribution with the finite first moment. Its
'bulk' scaling limit yields a distribution whose first moment reproduces the
well-known result of Chalker and Mehlig \cite{ChalkerMehlig1998}, and we
provide the 'edge' scaling distribution for this case as well. Our method
involves evaluating the ensemble average of products and ratios of integer and
half-integer powers of characteristic polynomials for Ginibre matrices, which
we perform in the framework of a supersymmetry approach. Our paper complements
recent studies by Bourgade and Dubach \cite{BourgadeDubach}.Comment: published versio
An Isocurvature CDM Cosmogony. II. Observational Tests
A companion paper presents a worked model for evolution through inflation to
initial conditions for an isocurvature model for structure formation. It is
shown here that the model is consistent with the available observational
constraints that can be applied without the help of numerical simulations. The
model gives an acceptable fit to the second moments of the angular fluctuations
in the thermal background radiation and the second through fourth moments of
the measured large-scale fluctuations in galaxy counts, within the possibly
significant uncertainties in these measurements. The cluster mass function
requires a rather low but observationally acceptable mass density,
0.1\lsim\Omega\lsim 0.2 in a cosmologically flat universe. Galaxies would be
assembled earlier in this model than in the adiabatic version, an arguably good
thing. Aspects of the predicted non-Gaussian character of the anisotropy of the
thermal background radiation in this model are discussed.Comment: 14 pages, 3 postscript figures, uses aas2pp4.st
The Ellipticity of the Disks of Spiral Galaxies
The disks of spiral galaxies are generally elliptical rather than circular.
The distribution of ellipticities can be fit with a log-normal distribution.
For a sample of 12,764 galaxies from the Sloan Digital Sky Survey Data Release
1 (SDSS DR1), the distribution of apparent axis ratios in the i band is best
fit by a log-normal distribution of intrinsic ellipticities with ln epsilon =
-1.85 +/- 0.89. For a sample of nearly face-on spiral galaxies, analyzed by
Andersen and Bershady using both photometric and spectroscopic data, the best
fitting distribution of ellipticities has ln epsilon = -2.29 +/- 1.04. Given
the small size of the Andersen-Bershady sample, the two distribution are not
necessarily inconsistent. If the ellipticity of the potential were equal to
that of the light distribution of the SDSS DR1 galaxies, it would produce 1.0
magnitudes of scatter in the Tully-Fisher relation, greater than is observed.
The Andersen-Bershady results, however, are consistent with a scatter as small
as 0.25 magnitudes in the Tully-Fisher relation.Comment: 19 pages, 5 figures; ApJ, accepte
Universal Results for Correlations of Characteristic Polynomials: Riemann-Hilbert Approach
We prove that general correlation functions of both ratios and products of
characteristic polynomials of Hermitian random matrices are governed by
integrable kernels of three different types: a) those constructed from
orthogonal polynomials; b) constructed from Cauchy transforms of the same
orthogonal polynomials and finally c) those constructed from both orthogonal
polynomials and their Cauchy transforms. These kernels are related with the
Riemann-Hilbert problem for orthogonal polynomials. For the correlation
functions we obtain exact expressions in the form of determinants of these
kernels. Derived representations enable us to study asymptotics of correlation
functions of characteristic polynomials via Deift-Zhou
steepest-descent/stationary phase method for Riemann-Hilbert problems, and in
particular to find negative moments of characteristic polynomials. This reveals
the universal parts of the correlation functions and moments of characteristic
polynomials for arbitrary invariant ensemble of symmetry class.Comment: 34page
Tools for Dissecting Supernova Remnants Observed with Chandra: Methods and Application to the Galactic Remnant W49B
We introduce methods to quantify the X-ray morphologies of supernova remnants
observed with the Chandra X-ray Telescope. These include a power-ratio
technique to measure morphological asymmetries, correlation-length analysis to
probe chemical segregation and distribution, and wavelet-transform analysis to
quantify X-ray substructure. We demonstrate the utility and accuracy of these
techniques on relevant synthetic data. Additionally, we show the methods'
capabilities by applying them to the 55-ks Chandra ACIS observation of the
galactic supernova remnant W49B. We analyze the images of prominent emission
lines in W49B and use the results to discern physical properties. We find that
the iron morphology is very distinct from the other elements: it is
statistically more asymmetric, more segregated, and has 25% larger emitting
substructures than the lighter ions. Comparatively, the silicon, sulfur, argon,
and calcium are well-mixed, more isotropic, and have smaller, equally-sized
emitting substructures. Based on fits of XMM-Newton spectra in regions
identified as iron rich and iron poor, we determine that the iron in W49B must
have been anisotropically ejected. We measure the abundance ratios in many
regions, and we find that large, local variations are persistent throughout the
remnant. We compare the mean, global abundance ratios to those predicted by
spherical and bipolar core-collapse explosions; the results are consistent with
a bipolar origin from a 25 solar mass progenitor. We calculate the filling
factor of iron from the volume of its emitting substructures, enabling more
precise mass estimates than previous studies. Overall, this work is a first
step toward rigorously describing the physical properties of supernova remnants
for comparison within and between sources.Comment: 51 pages, 24 figures, accepted by ApJ. For full resolution figures,
see http://www.astro.ucsc.edu/~lopez/paper.html Fixed typo in URL; no other
change
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