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1. CJM Online first
On the Local Convexity of Intersection Bodies of Revolution One of the fundamental results in Convex Geometry is Busemann's
theorem, which states that the intersection body of a symmetric convex
body is convex. Thus, it is only natural to ask if there is a
quantitative version of Busemann's theorem, i.e., if the intersection
body operation actually improves convexity. In this paper we
concentrate on the symmetric bodies of revolution to provide several
results on the (strict) improvement of convexity under the
intersection body operation. It is shown that the intersection body of
a symmetric convex body of revolution has the same asymptotic behavior
near the equator as the Euclidean
ball. We apply this result to show that in sufficiently high
dimension the double intersection body of a symmetric convex body of
revolution is very close to an ellipsoid in the Banach-Mazur
distance. We also prove results on the local convexity at the equator
of intersection bodies in the class of star bodies of revolution.
Keywords:convex bodies, intersection bodies of star bodies, Busemann's theorem, local convexity Categories:52A20, 52A38, 44A12 |
2. CJM 2012 (vol 66 pp. 700)
Inversion of the Radon Transform on the Free Nilpotent Lie Group of Step Two Let $F_{2n,2}$ be the free nilpotent Lie group of step two on $2n$
generators, and let $\mathbf P$ denote the affine automorphism group
of $F_{2n,2}$. In this article the theory of continuous wavelet
transform on $F_{2n,2}$ associated with $\mathbf P$ is developed,
and then a type of radial wavelets is constructed. Secondly, the
Radon transform on $F_{2n,2}$ is studied and two equivalent
characterizations of the range for Radon transform are given.
Several kinds of inversion Radon transform formulae
are established. One is obtained from the Euclidean Fourier transform, the others are from group Fourier transform. By using wavelet transform we deduce an inversion formula of the Radon
transform, which
does not require the smoothness of
functions if the wavelet satisfies the differentiability property.
Specially, if $n=1$, $F_{2,2}$ is the $3$-dimensional Heisenberg group $H^1$, the
inversion formula of the Radon transform is valid which is
associated with the sub-Laplacian on $F_{2,2}$. This result cannot
be extended to the case $n\geq 2$.
Keywords:Radon transform, wavelet transform, free nilpotent Lie group, unitary representation, inversion formula, sub-Laplacian Categories:43A85, 44A12, 52A38 |
3. CJM 2011 (vol 64 pp. 961)
Densities of Short Uniform Random Walks We study the densities of uniform random walks in the plane. A special focus
is on the case of short walks with three or four steps and less completely
those with five steps. As one of the main results, we obtain a hypergeometric
representation of the density for four steps, which complements the classical
elliptic representation in the case of three steps. It appears unrealistic
to expect similar results for more than five steps. New results are also
presented concerning the moments of uniform random walks and, in particular,
their derivatives. Relations with Mahler measures are discussed.
Keywords:random walks, hypergeometric functions, Mahler measure Categories:60G50, 33C20, 34M25, 44A10 |
4. CJM 2010 (vol 62 pp. 870)
The Brascamp-Lieb Polyhedron
A set of necessary and sufficient conditions for the Brascamp--Lieb inequality to hold has recently been found by Bennett, Carbery, Christ, and Tao. We present an analysis of these conditions. This analysis allows us to give a concise description of the set where the inequality holds in the case where each of the linear maps involved has co-rank $1$. This complements the result of Barthe concerning the case where the linear maps all have rank $1$. Pushing our analysis further, we describe the case where the maps have either rank $1$ or rank $2$. A separate but related problem is to give a list of the finite number of conditions necessary and sufficient for the Brascamp--Lieb inequality to hold. We present an algorithm which generates such a list.
Keywords:Brascamp-Lieb inequality, Loomis-Whitney inequality, lattice, flag Categories:44A35, 14M15, 26D20 |
5. CJM 2006 (vol 58 pp. 249)
Convergence of Fourier--PadÃ© Approximants for Stieltjes Functions We prove convergence of diagonal multipoint Pad\'e approximants of
Stieltjes-type functions when a certain moment problem is
determinate. This is used for the study of the convergence of
Fourier--Pad\'e and nonlinear Fourier--Pad\'e approximants for such
type of functions.
Keywords:rational approximation, multipoint PadÃ© approximants, Fourier--PadÃ© approximants, moment problem Categories:41A20, 41A21, 44A60 |
6. CJM 2005 (vol 57 pp. 941)
Some Transformations of Hausdorff Moment Sequences and Harmonic Numbers We introduce some non-linear transformations from the set of
Hausdorff moment sequences into itself; among
them is the one defined by
the formula:
$T((a_n)_n)=1/(a_0+\dots +a_n)$. We give some examples of
Hausdorff moment sequences arising from the transformations and
provide the corresponding measures: one of these sequences is the
reciprocal of the harmonic numbers $(1+1/2+\dots +1/(n+1))^{-1}$.
Categories:44A60, 40B05 |
7. CJM 1997 (vol 49 pp. 708)
Density questions for the truncated matrix moment problem For a truncated matrix moment problem, we describe in detail
the set of positive definite matrices of measures $\mu$ in $V_{2n}$ (this is
the set of solutions of the problem of degree $2n$) for which the polynomials
up to degree $n$ are dense in the corresponding space ${\cal L}^2(\mu)$.
These matrices of measures are exactly the extremal measures of the set $V_n$.
Categories:42C05, 44A60 |