3,650 research outputs found
Fully Analyzing an Algebraic Polya Urn Model
This paper introduces and analyzes a particular class of Polya urns: balls
are of two colors, can only be added (the urns are said to be additive) and at
every step the same constant number of balls is added, thus only the color
compositions varies (the urns are said to be balanced). These properties make
this class of urns ideally suited for analysis from an "analytic combinatorics"
point-of-view, following in the footsteps of Flajolet-Dumas-Puyhaubert, 2006.
Through an algebraic generating function to which we apply a multiple
coalescing saddle-point method, we are able to give precise asymptotic results
for the probability distribution of the composition of the urn, as well as
local limit law and large deviation bounds.Comment: LATIN 2012, Arequipa : Peru (2012
An Interesting Class of Operators with unusual Schatten-von Neumann behavior
We consider the class of integral operators Q_\f on of the form
(Q_\f f)(x)=\int_0^\be\f (\max\{x,y\})f(y)dy. We discuss necessary and
sufficient conditions on to insure that is bounded, compact,
or in the Schatten-von Neumann class \bS_p, . We also give
necessary and sufficient conditions for to be a finite rank
operator. However, there is a kind of cut-off at , and for membership in
\bS_{p}, , the situation is more complicated. Although we give
various necessary conditions and sufficient conditions relating to
Q_{\phi}\in\bS_{p} in that range, we do not have necessary and sufficient
conditions. In the most important case , we have a necessary condition and
a sufficient condition, using and modulus of continuity,
respectively, with a rather small gap in between. A second cut-off occurs at
: if \f is sufficiently smooth and decays reasonably fast, then \qf
belongs to the weak Schatten-von Neumann class \wS{1/2}, but never to
\bS_{1/2} unless \f=0.
We also obtain results for related families of operators acting on
and .
We further study operations acting on bounded linear operators on
related to the class of operators Q_\f. In particular we
study Schur multipliers given by functions of the form and
we study properties of the averaging projection (Hilbert-Schmidt projection)
onto the operators of the form Q_\f.Comment: 87 page
Upper tails for counting objects in randomly induced subhypergraphs and rooted random graphs
General upper tail estimates are given for counting edges in a random induced
subhypergraph of a fixed hypergraph H, with an easy proof by estimating the
moments. As an application we consider the numbers of arithmetic progressions
and Schur triples in random subsets of integers. In the second part of the
paper we return to the subgraph counts in random graphs and provide upper tail
estimates in the rooted case.Comment: 15 page
Functional characterization and structure-guided mutational analysis of the transsulfuration enzyme cystathionine Îł-lyase from toxoplasma gondii
Sulfur-containing amino acids play essential roles in many organisms. The protozoan parasite Toxoplasma gondii includes the genes for cystathionine ÎČ-synthase and cystathionine Îł-lyase (TgCGL), as well as for cysteine synthase, which are crucial enzymes of the transsulfuration and de novo pathways for cysteine biosynthesis, respectively. These enzymes are specifically expressed in the oocyst stage of T. gondii. However, their functionality has not been investigated. Herein, we expressed and characterized the putative CGL from T. gondii. Recombinant TgCGL almost exclusively catalyses the α,Îł-hydrolysis of L-cystathionine to form L-cysteine and displays marginal reactivity toward L-cysteine. Structure-guided homology modelling revealed two striking amino acid differences between the human and parasite CGL active-sites (Glu59 and Ser340 in human to Ser77 and Asn360 in toxoplasma). Mutation of Asn360 to Ser demonstrated the importance of this residue in modulating the specificity for the catalysis of α,ÎČ-versus α,Îł-elimination of L-cystathionine. Replacement of Ser77 by Glu completely abolished activity towards L-cystathionine. Our results suggest that CGL is an important functional enzyme in T. gondii, likely implying that the reverse transsulfuration pathway is operative in the parasite; we also probed the roles of active-site architecture and substrate binding conformations as determinants of reaction specificity in transsulfuration enzymes
Monotone graph limits and quasimonotone graphs
The recent theory of graph limits gives a powerful framework for
understanding the properties of suitable (convergent) sequences of
graphs in terms of a limiting object which may be represented by a symmetric
function on , i.e., a kernel or graphon. In this context it is
natural to wish to relate specific properties of the sequence to specific
properties of the kernel. Here we show that the kernel is monotone (i.e.,
increasing in both variables) if and only if the sequence satisfies a
`quasi-monotonicity' property defined by a certain functional tending to zero.
As a tool we prove an inequality relating the cut and norms of kernels of
the form with and monotone that may be of interest in its
own right; no such inequality holds for general kernels.Comment: 38 page
Searching for young Jupiter analogs around AP Col: L-band high-contrast imaging of the closest pre-main sequence star
The nearby M-dwarf AP Col was recently identified by Riedel et al. 2011 as a
pre-main sequence star (age 12 - 50 Myr) situated only 8.4 pc from the Sun. The
combination of its youth, distance, and intrinsically low luminosity make it an
ideal target to search for extrasolar planets using direct imaging. We report
deep adaptive optics observations of AP Col taken with VLT/NACO and Keck/NIRC2
in the L-band. Using aggressive speckle suppression and background subtraction
techniques, we are able to rule out companions with mass m >= 0.5 - 1M_Jup for
projected separations a>4.5 AU, and m >= 2 M_Jup for projected separations as
small as 3 AU, assuming an age of 40 Myr using the COND theoretical
evolutionary models. Using a different set of models the mass limits increase
by a factor of ~2. The observations presented here are the deepest
mass-sensitivity limits yet achieved within 20 AU on a star with direct
imaging. While Doppler radial velocity surveys have shown that Jovian bodies
with close-in orbits are rare around M-dwarfs, gravitational microlensing
studies predict that ~17% of these stars host massive planets with orbital
separations of 1-10 AU. Sensitive high-contrast imaging observations, like
those presented here, will help to validate results from complementary
detection techniques by determining the frequency of gas giant planets on wide
orbits around M-dwarfs.Comment: Accepted for publication in ApJ, 6 pages text ApJ style (incl.
references), 4 figures, 1 tabl
Rotordynamic coefficients and leakage flow of parallel grooved seals and smooth seals
Based on Childs finite length solution for annular plain seals an extension of the bulk flow theory is derived to calculate the rotordynamic coefficients and the leakage flow of seals with parallel grooves in the stator. Hirs turbulent lubricant equations are modified to account for the different friction factors in circumferential and axial direction. Furthermore an average groove depth is introduced to consider the additional circumferential flow in the grooves. Theoretical and experimental results are compared for the smooth constant clearance seal and the corresponding seal with parallel grooves. Compared to the smooth seal the direct and cross-coupled stiffness coefficients as well as the direct damping coefficients are lower in the grooved seal configuration. Leakage is reduced by the grooving pattern
Peculiar long-range superexchange in Cu2A2O7 (A = P, As, V) as a key element of the microscopic magnetic model
A microscopic magnetic model for alpha-Cu2P2O7 is evaluated in a combined
theoretical and experimental study. Despite a dominant intradimer coupling J1,
sizable interdimer couplings enforce long-range magnetic ordering at T_N=27 K.
The spin model for alpha-Cu2P2O7 is compared to the models of the isostructural
beta-Cu2V2O7 and alpha-Cu2As2O7 systems. As a surprise, coupled dimers in
alpha-Cu2P2O7 and alternating chains in alpha-Cu2As2O7 contrast with a
honeycomb lattice in beta-Cu2V2O7. We find that the qualitative difference in
the coupling regime of these isostructural compounds is governed by the nature
of AO4 side groups: d-elements (A = V) hybridize with nearby O atoms forming a
Cu-O-A-O-Cu superexchange path, while for p-elements (A = P, As) the
superexchange is realized via O-O edges of the tetrahedron. Implications for a
broad range of systems are discussed.Comment: 8 pages, 5 figures, 1 table; discussion extende
Detection of the nearest Jupiter analog in radial velocity and astrometry data
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.The presence of Jupiter is crucial to the architecture of the Solar System and models underline this to be a generic feature of planetary systems. We find the detection of the difference between the position and motion recorded by the contemporary astrometric satellite Gaia and its precursor Hipparcos can be used to discover Jupiter-like planets. We illustrate how observations of the nearby star Indi A giving astrometric and radial velocity data can be used to independently find the orbit of its suspected companion. The radial velocity and astrometric data provide complementary detections which allow for a much stronger solution than either technique would provide individually. We quantify Indi A b as the closest Jupiter-like exoplanet with a mass of 3 on a slightly eccentric orbit with an orbital period of 45 yr. While other long-period exoplanets have been discovered, Indi A b provides a well constrained mass and along with the well-studied brown dwarf binary in orbit around Indi A means that the system provides a benchmark case for our understanding of the formation of gas giant planets and brown dwarfs.Peer reviewe
AKLT Models with Quantum Spin Glass Ground States
We study AKLT models on locally tree-like lattices of fixed connectivity and
find that they exhibit a variety of ground states depending upon the spin,
coordination and global (graph) topology. We find a) quantum paramagnetic or
valence bond solid ground states, b) critical and ordered N\'eel states on
bipartite infinite Cayley trees and c) critical and ordered quantum vector spin
glass states on random graphs of fixed connectivity. We argue, in consonance
with a previous analysis, that all phases are characterized by gaps to local
excitations. The spin glass states we report arise from random long ranged
loops which frustrate N\'eel ordering despite the lack of randomness in the
coupling strengths.Comment: 10 pages, 1 figur
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