293 research outputs found
Spatiospectral concentration on a sphere
We pose and solve the analogue of Slepian's time-frequency concentration
problem on the surface of the unit sphere to determine an orthogonal family of
strictly bandlimited functions that are optimally concentrated within a closed
region of the sphere, or, alternatively, of strictly spacelimited functions
that are optimally concentrated within the spherical harmonic domain. Such a
basis of simultaneously spatially and spectrally concentrated functions should
be a useful data analysis and representation tool in a variety of geophysical
and planetary applications, as well as in medical imaging, computer science,
cosmology and numerical analysis. The spherical Slepian functions can be found
either by solving an algebraic eigenvalue problem in the spectral domain or by
solving a Fredholm integral equation in the spatial domain. The associated
eigenvalues are a measure of the spatiospectral concentration. When the
concentration region is an axisymmetric polar cap the spatiospectral projection
operator commutes with a Sturm-Liouville operator; this enables the
eigenfunctions to be computed extremely accurately and efficiently, even when
their area-bandwidth product, or Shannon number, is large. In the asymptotic
limit of a small concentration region and a large spherical harmonic bandwidth
the spherical concentration problem approaches its planar equivalent, which
exhibits self-similarity when the Shannon number is kept invariant.Comment: 48 pages, 17 figures. Submitted to SIAM Review, August 24th, 200
Potential anomalies on a sphere: Applications to the thickness of the lunar crust
International audienc
A Serenitatis origin for the Imbrian grooves and South Pole-Aitken thorium anomaly
International audienc
The “Procellarum KREEP Terrane”: Implications for mare volcanism and lunar evolution
International audienc
Thickness of the Martian crust: Improved constraints from geoid-to-topography ratios
International audienc
Nonuniform cratering of the Moon and a revised crater chronology of the inner solar system
International audienceâ–ş We model the cratering of the Moon and terrestrial planets. â–ş We account for cratering asymmetries and megaregolith. â–ş We revise the crater chronology method. â–ş We give new age estimates of key planetary surface
Major lunar crustal terranes: Surface expressions and crust-mantle origins
International audienc
Pre-mRNA splicing in higher plants.
P re-mRNA splicing is one of the fundamental processes in constitutive and regulated gene expression in eukaryotes. During splicing, introns present in primary gene transcripts are removed and exons are ligated to produce translationally competent mRNAs. The basic mechanism of intron excision is similar in all eukaryotes. The reaction is mediated by the spliceosome, a large ribonucleoprotein (RNP) complex, which is assembled anew at each intron from small nuclear RNP particles (U-snRNPs) and numerous protein factors. Spliceosome assembly is a highly ordered and dynamic reaction, involving hydrolysis of several ATP molecules and many structural rearrangements Properties of plant introns The intron and exon organization of higher plant genes is similar to that of vertebrates In spite of these similarities, the requirements for intron recognition in plants differ from those in other eukaryotes, and plant cells generally fail to splice heterologous pre-mRNAs. The most important difference is a strong compositional bias for UA-or U-rich sequences in plant introns compared with those from yeast and vertebrates U12-type introns A minor class of nuclear pre-mRNA introns, referred to as U12-type or AT-AC introns (because they frequently start with AT and terminate with AC) have recently been described 3,13 . These introns contain different splice site and branch point sequences, and are excised by an alternative U12-type spliceosom
- …