573 research outputs found
Four types of special functions of G_2 and their discretization
Properties of four infinite families of special functions of two real
variables, based on the compact simple Lie group G2, are compared and
described. Two of the four families (called here C- and S-functions) are well
known, whereas the other two (S^L- and S^S-functions) are not found elsewhere
in the literature. It is shown explicitly that all four families have similar
properties. In particular, they are orthogonal when integrated over a finite
region F of the Euclidean space, and they are discretely orthogonal when their
values, sampled at the lattice points F_M \subset F, are added up with a weight
function appropriate for each family. Products of ten types among the four
families of functions, namely CC, CS, SS, SS^L, CS^S, SS^L, SS^S, S^SS^S,
S^LS^S and S^LS^L, are completely decomposable into the finite sum of the
functions. Uncommon arithmetic properties of the functions are pointed out and
questions about numerous other properties are brought forward.Comment: 18 pages, 4 figures, 4 table
Four dimensional Lie symmetry algebras and fourth order ordinary differential equations
Realizations of four dimensional Lie algebras as vector fields in the plane
are explicitly constructed. Fourth order ordinary differential equations which
admit such Lie symmetry algebras are derived. The route to their integration is
described.Comment: 12 page
Certified Rapid Solution of Parametrized Linear Elliptic Equations: Application to Parameter Estimation
We present a technique for the rapid and reliable
evaluation of linear-functional output of elliptic partial differential equations with affine parameter dependence. The essential components are (i) rapidly uniformly convergent reduced-basis approximations — Galerkin projection onto a space WN spanned by solutions of the governing partial differential equation at N (optimally) selected points in parameter space; (ii) a posteriori error estimation — relaxations of the residual equation that provide inexpensive yet sharp and rigorous bounds for the error in the outputs; and (iii) offline/online computational procedures — stratagems that exploit affine parameter dependence to de-couple the generation and projection stages of the approximation
process. The operation count for the online stage — in which, given a new parameter value, we calculate the output and associated error bound — depends only on N (typically small) and the parametric complexity of the problem. The method is thus ideally suited to the many-query and real-time contexts. In this paper, based on the technique we develop a robust inverse
computational method for very fast solution of inverse problems characterized by parametrized partial differential equations. The essential ideas are in three-fold: first, we apply the technique to the forward problem for the rapid certified evaluation of PDE input-output relations and associated rigorous error bounds; second, we incorporate the reduced-basis approximation and
error bounds into the inverse problem formulation; and third, rather than regularize the goodness-of-fit objective, we may instead identify all (or almost all, in the probabilistic sense)
system configurations consistent with the available experimental data — well-posedness is reflected in a bounded "possibility region" that furthermore shrinks as the experimental error is
decreased.Singapore-MIT Alliance (SMA
Cavity Light Bullets: 3D Localized Structures in a Nonlinear Optical Resonator
We consider the paraxial model for a nonlinear resonator with a saturable
absorber beyond the mean-field limit and develop a method to study the
modulational instabilities leading to pattern formation in all three spatial
dimensions. For achievable parametric domains we observe total radiation
confinement and the formation of 3D localised bright structures. At difference
from freely propagating light bullets, here the self-organization proceeds from
the resonator feedback, combined with diffraction and nonlinearity. Such
"cavity" light bullets can be independently excited and erased by appropriate
pulses, and once created, they endlessly travel the cavity roundtrip. Also, the
pulses can shift in the transverse direction, following external field
gradients.Comment: 4 pages, 3 figures, simulations files available at
http://www.ba.infn.it/~maggipin/PRLmovies.htm, submitted to Physical Review
Letters on 24 March 200
The rings of n-dimensional polytopes
Points of an orbit of a finite Coxeter group G, generated by n reflections
starting from a single seed point, are considered as vertices of a polytope
(G-polytope) centered at the origin of a real n-dimensional Euclidean space. A
general efficient method is recalled for the geometric description of G-
polytopes, their faces of all dimensions and their adjacencies. Products and
symmetrized powers of G-polytopes are introduced and their decomposition into
the sums of G-polytopes is described. Several invariants of G-polytopes are
found, namely the analogs of Dynkin indices of degrees 2 and 4, anomaly numbers
and congruence classes of the polytopes. The definitions apply to
crystallographic and non-crystallographic Coxeter groups. Examples and
applications are shown.Comment: 24 page
On character generators for simple Lie algebras
We study character generating functions (character generators) of simple Lie
algebras. The expression due to Patera and Sharp, derived from the Weyl
character formula, is first reviewed. A new general formula is then found. It
makes clear the distinct roles of ``outside'' and ``inside'' elements of the
integrity basis, and helps determine their quadratic incompatibilities. We
review, analyze and extend the results obtained by Gaskell using the Demazure
character formulas. We find that the fundamental generalized-poset graphs
underlying the character generators can be deduced from such calculations.
These graphs, introduced by Baclawski and Towber, can be simplified for the
purposes of constructing the character generator. The generating functions can
be written easily using the simplified versions, and associated Demazure
expressions. The rank-two algebras are treated in detail, but we believe our
results are indicative of those for general simple Lie algebras.Comment: 50 pages, 11 figure
Three dimensional C-, S- and E-transforms
Three dimensional continuous and discrete Fourier-like transforms, based on
the three simple and four semisimple compact Lie groups of rank 3, are
presented. For each simple Lie group, there are three families of special
functions (-, -, and -functions) on which the transforms are built.
Pertinent properties of the functions are described in detail, such as their
orthogonality within each family, when integrated over a finite region of
the 3-dimensional Euclidean space (continuous orthogonality), as well as when
summed up over a lattice grid (discrete orthogonality). The
positive integer sets up the density of the lattice containing . The
expansion of functions given either on or on is the paper's main
focus.Comment: 24 pages, 13 figure
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