7,429 research outputs found
Continuum Singularities of a Mean Field Theory of Collisions
Consider a complex energy for a -particle Hamiltonian and let
be any wave packet accounting for any channel flux. The time independent
mean field (TIMF) approximation of the inhomogeneous, linear equation
consists in replacing by a product or Slater
determinant of single particle states This results, under the
Schwinger variational principle, into self consistent TIMF equations
in single particle space. The method is a
generalization of the Hartree-Fock (HF) replacement of the -body homogeneous
linear equation by single particle HF diagonalizations
We show how, despite strong nonlinearities in this mean
field method, threshold singularities of the {\it inhomogeneous} TIMF equations
are linked to solutions of the {\it homogeneous} HF equations.Comment: 21 pages, 14 figure
From "Dirac combs" to Fourier-positivity
Motivated by various problems in physics and applied mathematics, we look for
constraints and properties of real Fourier-positive functions, i.e. with
positive Fourier transforms. Properties of the "Dirac comb" distribution and of
its tensor products in higher dimensions lead to Poisson resummation, allowing
for a useful approximation formula of a Fourier transform in terms of a limited
number of terms. A connection with the Bochner theorem on positive definiteness
of Fourier-positive functions is discussed. As a practical application, we find
simple and rapid analytic algorithms for checking Fourier-positivity in 1- and
(radial) 2-dimensions among a large variety of real positive functions. This
may provide a step towards a classification of positive positive-definite
functions.Comment: 17 pages, 14 eps figures (overall 8 figures in the text
On the positivity of Fourier transforms
Characterizing in a constructive way the set of real functions whose Fourier
transforms are positive appears to be yet an open problem. Some sufficient
conditions are known but they are far from being exhaustive. We propose two
constructive sets of necessary conditions for positivity of the Fourier
transforms and test their ability of constraining the positivity domain. One
uses analytic continuation and Jensen inequalities and the other deals with
Toeplitz determinants and the Bochner theorem. Applications are discussed,
including the extension to the two-dimensional Fourier-Bessel transform and the
problem of positive reciprocity, i.e. positive functions with positive
transforms.Comment: 12 pages, 9 figures (in 4 groups
On positive functions with positive Fourier transforms
Using the basis of Hermite-Fourier functions (i.e. the quantum oscillator
eigenstates) and the Sturm theorem, we derive the practical constraints for a
function and its Fourier transform to be both positive. We propose a
constructive method based on the algebra of Hermite polynomials. Applications
are extended to the 2-dimensional case (i.e. Fourier-Bessel transforms and the
algebra of Laguerre polynomials) and to adding constraints on derivatives, such
as monotonicity or convexity.Comment: 12 pages, 23 figures. High definition figures can be obtained upon
request at [email protected] or [email protected]
Elementary Derivative Tasks and Neural Net Multiscale Analysis of Tasks
Neural nets are known to be universal approximators. In particular, formal
neurons implementing wavelets have been shown to build nets able to approximate
any multidimensional task. Such very specialized formal neurons may be,
however, difficult to obtain biologically and/or industrially. In this paper we
relax the constraint of a strict ``Fourier analysis'' of tasks. Rather, we use
a finite number of more realistic formal neurons implementing elementary tasks
such as ``window'' or ``Mexican hat'' responses, with adjustable widths. This
is shown to provide a reasonably efficient, practical and robust,
multifrequency analysis. A training algorithm, optimizing the task with respect
to the widths of the responses, reveals two distinct training modes. The first
mode induces some of the formal neurons to become identical, hence promotes
``derivative tasks''. The other mode keeps the formal neurons distinct.Comment: latex neurondlt.tex, 7 files, 6 figures, 9 pages [SPhT-T01/064],
submitted to Phys. Rev.
Existence of a Density Functional for an Intrinsic State
A generalization of the Hohenberg-Kohn theorem proves the existence of a
density functional for an intrinsic state, symmetry violating, out of which a
physical state with good quantum numbers can be projected.Comment: 6 page
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