3,440 research outputs found
Learning with Algebraic Invariances, and the Invariant Kernel Trick
When solving data analysis problems it is important to integrate prior
knowledge and/or structural invariances. This paper contributes by a novel
framework for incorporating algebraic invariance structure into kernels. In
particular, we show that algebraic properties such as sign symmetries in data,
phase independence, scaling etc. can be included easily by essentially
performing the kernel trick twice. We demonstrate the usefulness of our theory
in simulations on selected applications such as sign-invariant spectral
clustering and underdetermined ICA
Decoherence and Entropy Production in Relativistic Nuclear Collisions
Short thermalization times of less than 1 fm/c for quark and gluon matter
have been suggested by recent experiments at the Relativistic Heavy Ion
Collider (RHIC). It has been difficult to justify this rapid thermalization in
first-principle calculations based on perturbation theory or the color glass
condensate picture. Here, we address the related question of the decoherence of
the gluon field, which is a necessary component of thermalization. We present a
simplified leading-order computation of the decoherence time of a gluon
ensemble subject to an incoming flux of Weizsacker-Williams gluons. We also
discuss the entropy produced during the decoherence process and its relation to
the entropy in the final state which has been measured experimentally.Comment: 8 pages, 3 figure
Benchmarking a semiclassical impurity solver for dynamical-mean-field theory: self-energies and magnetic transitions of the single-orbital Hubbard model
An investigation is presented of the utility of semiclassical approximations
for solving the quantum-impurity problems arising in the dynamical-mean-field
approach to the correlated-electron models. The method is based on performing a
exact numerical integral over the zero-Matsubara-frequency component of the
spin part of a continuous Hubbard-Stratonovich field, along with a
spin-field-dependent steepest descents treatment of the charge part. We test
this method by applying it to one or two site approximations to the single band
Hubbard model with different band structures, and comparing the results to
quantum Monte-Carlo and simplified exact diagonalization calculations. The
resulting electron self-energies, densities of states and magnetic transition
temperatures show reasonable agreement with the quantum Monte-Carlo simulation
over wide parameter ranges, suggesting that the semiclassical method is useful
for obtaining a reasonable picture of the physics in situations where other
techniques are too expensive.Comment: 14 pages, 15 figure
Platelet lysate as a serum substitute for 2D static and 3D perfusion culture of stromal vascular fraction cells from human adipose tissue
Fetal bovine serum (FBS) and fibroblast growth factor (FGF)-2 are key supplements for the culture of stromal vascular fraction (SVF) cells from adipose tissue, both for typical monolayer (2D) expansion and for streamlined generation of osteogenic-vasculogenic grafts in 3D perfusion culture. The present study investigates whether factors present in human platelet lysate (PL) could substitute for FBS and FGF-2 in 2D and 3D culture models of SVF cells from human lipoaspirates. SVF cells were grown in medium supplemented with 10% FBS+FGF-2 or with 5% PL. In 2D cultures, PL initially supported SVF cell proliferation, but resulted in growth arrest shortly after the first passage. Freshly isolated SVF cells cultured with both media under perfusion for 5 days within 3D ceramic scaffolds induced bone formation after subcutaneous implantation in nude mice. However, blood vessels of donor origin were generated only using FBS+FGF-2-cultured cells. This was unexpected, because the proportion of CD34+/CD31+ endothelial lineage cells was significantly higher with PL than that of FBS+FGF-2 (33% vs. 3%, respectively). These results support the use of PL as a substitute of FBS+FGF-2 for short-term culture of human SVF cells, and indicate that more specific serum-free formulations are required to maintain a functionally vasculogenic fraction of SVF cells expanded under 3D perfusion
Probing molecular free energy landscapes by periodic loading
Single molecule pulling experiments provide information about interactions in
biomolecules that cannot be obtained by any other method. However, the
reconstruction of the molecule's free energy profile from the experimental data
is still a challenge, in particular for the unstable barrier regions. We
propose a new method for obtaining the full profile by introducing a periodic
ramp and using Jarzynski's identity for obtaining equilibrium quantities from
non-equilibrium data. Our simulated experiments show that this method delivers
significant more accurate data than previous methods, under the constraint of
equal experimental effort.Comment: 4 pages, 3 figure
Imaging the Electrostatic Potential of Transmembrane Channels: Atomic Probe Microscopy of OmpF Porin
This is the published version. Copyright 2002 by Elsevier.The atomic force microscope (AFM) was used to image native OmpF porin and to detect the electrostatic potential generated by the protein. To this end the OmpF porin trimers from Escherichia coli was reproducibly imaged at a lateral resolution of ∼0.5 nm and a vertical resolution of ∼0.1 nm at variable electrolyte concentrations of the buffer solution. At low electrolyte concentrations the charged AFM probe not only contoured structural details of the membrane protein surface but also interacted with local electrostatic potentials. Differences measured between topographs recorded at variable ionic strength allowed mapping of the electrostatic potential of OmpF porin. The potential map acquired by AFM showed qualitative agreement with continuum electrostatic calculations based on the atomic OmpF porin embedded in a lipid bilayer at the same electrolyte concentrations. Numerical simulations of the experimental conditions showed the measurements to be reproduced quantitatively when the AFM probe was included in the calculations. This method opens a novel avenue to determine the electrostatic potential of native protein surfaces at a lateral resolution better than 1 nm and a vertical resolution of ∼0.1 nm
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