6,571 research outputs found
Exact Half-BPS Flux Solutions in M-theory with Symmetry: Local Solutions
We construct local solutions to 11-dimensional supergravity (or M-theory),
which are invariant under the superalgebra for all values of the parameter . The BPS constraints are reduced to a
single linear PDE on a complex function . The PDE was solved in 0806.0605
modulo application of boundary and regularity conditions. The physical fields
of the solutions are determined by , a harmonic function , and the
complex function . and are both functions on a 2-dimensional
compact Riemannian manifold. The harmonic function is freely chosen. We
obtain the expressions for the metric and the field strength in terms of ,
, and and show that these are indeed valid solutions of the Einstein,
Maxwell, and Bianchi equations. Finally we give a construction of one parameter
deformations of and as a function of
Packing Returning Secretaries
We study online secretary problems with returns in combinatorial packing
domains with candidates that arrive sequentially over time in random order.
The goal is to accept a feasible packing of candidates of maximum total value.
In the first variant, each candidate arrives exactly twice. All arrivals
occur in random order. We propose a simple 0.5-competitive algorithm that can
be combined with arbitrary approximation algorithms for the packing domain,
even when the total value of candidates is a subadditive function. For
bipartite matching, we obtain an algorithm with competitive ratio at least
for growing , and an algorithm with ratio at least
for all . We extend all algorithms and ratios to arrivals
per candidate.
In the second variant, there is a pool of undecided candidates. In each
round, a random candidate from the pool arrives. Upon arrival a candidate can
be either decided (accept/reject) or postponed (returned into the pool). We
mainly focus on minimizing the expected number of postponements when computing
an optimal solution. An expected number of is always
sufficient. For matroids, we show that the expected number can be reduced to
, where is the minimum of the ranks of matroid and
dual matroid. For bipartite matching, we show a bound of , where
is the size of the optimum matching. For general packing, we show a lower
bound of , even when the size of the optimum is .Comment: 23 pages, 5 figure
Nearly optimal solutions for the Chow Parameters Problem and low-weight approximation of halfspaces
The \emph{Chow parameters} of a Boolean function
are its degree-0 and degree-1 Fourier coefficients. It has been known
since 1961 (Chow, Tannenbaum) that the (exact values of the) Chow parameters of
any linear threshold function uniquely specify within the space of all
Boolean functions, but until recently (O'Donnell and Servedio) nothing was
known about efficient algorithms for \emph{reconstructing} (exactly or
approximately) from exact or approximate values of its Chow parameters. We
refer to this reconstruction problem as the \emph{Chow Parameters Problem.}
Our main result is a new algorithm for the Chow Parameters Problem which,
given (sufficiently accurate approximations to) the Chow parameters of any
linear threshold function , runs in time \tilde{O}(n^2)\cdot
(1/\eps)^{O(\log^2(1/\eps))} and with high probability outputs a
representation of an LTF that is \eps-close to . The only previous
algorithm (O'Donnell and Servedio) had running time \poly(n) \cdot
2^{2^{\tilde{O}(1/\eps^2)}}.
As a byproduct of our approach, we show that for any linear threshold
function over , there is a linear threshold function which
is \eps-close to and has all weights that are integers at most \sqrt{n}
\cdot (1/\eps)^{O(\log^2(1/\eps))}. This significantly improves the best
previous result of Diakonikolas and Servedio which gave a \poly(n) \cdot
2^{\tilde{O}(1/\eps^{2/3})} weight bound, and is close to the known lower
bound of (1/\eps)^{\Omega(\log \log (1/\eps))}\} (Goldberg,
Servedio). Our techniques also yield improved algorithms for related problems
in learning theory
Clustering of fermionic truncated expectation values via functional integration
I give a simple proof that the correlation functions of many-fermion systems
have a convergent functional Grassmann integral representation, and use this
representation to show that the cumulants of fermionic quantum statistical
mechanics satisfy l^1-clustering estimates
Rectification in Luttinger liquids
We investigate the rectification of an ac bias in Luttinger liquids in the
presence of an asymmetric potential (the ratchet effect). We show that strong
repulsive electron interaction enhances the ratchet current in comparison with
Fermi liquid systems, and the I-V curve is strongly asymmetric in the
low-voltage regime even for a weak asymmetric potential. At higher voltages the
ratchet current exhibits an oscillatory voltage dependence.Comment: 5 pages, Revte
Phyllosilicate Transitions in Ferromagnesian Soils: Short-Range Order Materials and Smectites Dominate Secondary Phases
Analyses of X-ray diffraction (XRD) patterns taken by the CheMin instrument on the Curiosity Rover in Gale crater have documented the presence of clay minerals interpreted as smectites and a suite of amorphous to short-range order materials termed X-ray amorphous materials. These X-ray amorphous materials are commonly ironrich and aluminum poor and likely some of them are weathering products rather than primary glasses due to the presence of volatiles. Outstanding questions remain regarding the chemical composition and mineral structure of these X-ray amorphous materials and the smectites present at Gale crater and what they indicate about environmental conditions during their formation. To gain a better understanding of the mineral transitions that occur within ferromagnesian parent materials, we have investigated the development of secondary clay minerals and shortrange order materials in two soil chronosequences with varying climates developing on ultramafic bedrock. Field Sites: We investigated soil weathering within two field locations, the Klamath Mountains of Northern California, and the Tablelands of Newfoundland, Canada. Both sites possess age dated or correlated recently deglaciated soils and undated but substantially older soils. In the Klamath mountains the Trinity Ultramafic Body was deglaciated roughly 15,000 years bp while in the Tablelands a moraine was dated to about 17,600 years bp. The Klamath Mountains feature a seasonally wet and dry climate while the Tablelands are wet year-round with saturated soil conditions observed during sampling and standing water observed within 3 of 4 soil pit sampling locations
Interferon-Γ inhibits DNA synthesis and insulin-like growth factor-II expression in human neuroblastoma cells
Interferon-Γ (IFN-Γ) is known to be an antiproliferative, differentiation agent in many cell types, including neuroblastoma. In this study, we determined the effects of IFN-Γ on cellular growth and expression of insulin-like growth factor II (IGF-II) and IGF receptors in the human neuroblastoma cell line SH-SY5Y. Incubation of SH-SY5Y cells in IFN-Γ (20–100 U/ml) induced the formation of long neuritic processes. IFN-Γ treatment also induced decreases in [ 3 H]TdR incorporation, as well as serum-dependent changes in cell number. Treatment with IFN-Γ reduced cell number 33% in the presence of serum but had no effect on cell number in the absence of serum. IGF-II mRNA content was 60% inhibited by IFN-Γ, and was not serum dependent. The concentration of immunoreactive IGF-II in SH-SY5Y conditioned medium was also reduced in the presence of IFN-Γ, to less than half of control levels. In contrast, type I IGF receptor mRNA content was increased more than three-fold after treatment with IFN-Γ and serum. Co-incubation in IFN-Γ (20–100 U/ml) and IGF-II on (3–10 nM) prevented the inhibitory effects of IFN-Γ on [ 3 H]TdR ncorporation in serum-free media. Our results suggest that IFN-Γ may inhibit DNA synthesis and cell growth by interfering with an IGF-II/type I IGF receptor autocrine growth or survival mechanism.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50226/1/490340502_ftp.pd
R-parity Conservation via the Stueckelberg Mechanism: LHC and Dark Matter Signals
We investigate the connection between the conservation of R-parity in
supersymmetry and the Stueckelberg mechanism for the mass generation of the B-L
vector gauge boson. It is shown that with universal boundary conditions for
soft terms of sfermions in each family at the high scale and with the
Stueckelberg mechanism for generating mass for the B-L gauge boson present in
the theory, electric charge conservation guarantees the conservation of
R-parity in the minimal B-L extended supersymmetric standard model. We also
discuss non-minimal extensions. This includes extensions where the gauge
symmetries arise with an additional U(1)_{B-L} x U(1)_X, where U(1)_X is a
hidden sector gauge group. In this case the presence of the additional U(1)_X
allows for a Z' gauge boson mass with B-L interactions to lie in the sub-TeV
region overcoming the multi-TeV LEP constraints. The possible tests of the
models at colliders and in dark matter experiments are analyzed including
signals of a low mass Z' resonance and the production of spin zero bosons and
their decays into two photons. In this model two types of dark matter
candidates emerge which are Majorana and Dirac particles. Predictions are made
for a possible simultaneous observation of new physics events in dark matter
experiments and at the LHC.Comment: 38 pages, 7 fig
Alice: The Rosetta Ultraviolet Imaging Spectrograph
We describe the design, performance and scientific objectives of the
NASA-funded ALICE instrument aboard the ESA Rosetta asteroid flyby/comet
rendezvous mission. ALICE is a lightweight, low-power, and low-cost imaging
spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy. It will
be the first UV spectrograph to study a comet at close range. It is designed to
obtain spatially-resolved spectra of Rosetta mission targets in the 700-2050 A
spectral band with a spectral resolution between 8 A and 12 A for extended
sources that fill its ~0.05 deg x 6.0 deg field-of-view. ALICE employs an
off-axis telescope feeding a 0.15-m normal incidence Rowland circle
spectrograph with a concave holographic reflection grating. The imaging
microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr
and CsI) and employs a 2 D delay-line readout array. The instrument is
controlled by an internal microprocessor. During the prime Rosetta mission,
ALICE will characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus,
and the nucleus/coma coupling; during cruise to the comet, ALICE will make
observations of the mission's two asteroid flyby targets and of Mars, its
moons, and of Earth's moon. ALICE has already successfully completed the
in-flight commissioning phase and is operating normally in flight. It has been
characterized in flight with stellar flux calibrations, observations of the
Moon during the first Earth fly-by, and observations of comet Linear T7 in 2004
and comet 9P/Tempel 1 during the 2005 Deep Impact comet-collision observing
campaignComment: 11 pages, 7 figure
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