11,482 research outputs found
A Metrizable Topology on the Contracting Boundary of a Group
The 'contracting boundary' of a proper geodesic metric space consists of
equivalence classes of geodesic rays that behave like rays in a hyperbolic
space. We introduce a geometrically relevant, quasi-isometry invariant topology
on the contracting boundary. When the space is the Cayley graph of a finitely
generated group we show that our new topology is metrizable.Comment: v1: 26 pages, 3 figures; v2: 44 pages, 6 figures, additional results;
v3: 46 pages, 7 figures, minor change
Stellar Differential Rotation and Coronal Timescales
We investigate the timescales of evolution of stellar coronae in response to
surface differential rotation and diffusion. To quantify this we study both the
formation time and lifetime of a magnetic flux rope in a decaying bipolar
active region. We apply a magnetic flux transport model to prescribe the
evolution of the stellar photospheric field, and use this to drive the
evolution of the coronal magnetic field via a magnetofrictional technique.
Increasing the differential rotation (i.e. decreasing the equator-pole lap
time) decreases the flux rope formation time. We find that the formation time
is dependent upon the geometric mean of the lap time and the surface diffusion
timescale. In contrast, the lifetime of flux ropes are proportional to the lap
time. With this, flux ropes on stars with a differential rotation of more than
eight times the solar value have a lifetime of less than two days. As a
consequence, we propose that features such as solar-like quiescent prominences
may not be easily observable on such stars, as the lifetimes of the flux ropes
which host the cool plasma are very short. We conclude that such high
differential rotation stars may have very dynamical coronae
Electronic structure of periodic curved surfaces -- topological band structure
Electronic band structure for electrons bound on periodic minimal surfaces is
differential-geometrically formulated and numerically calculated. We focus on
minimal surfaces because they are not only mathematically elegant (with the
surface characterized completely in terms of "navels") but represent the
topology of real systems such as zeolites and negative-curvature fullerene. The
band structure turns out to be primarily determined by the topology of the
surface, i.e., how the wavefunction interferes on a multiply-connected surface,
so that the bands are little affected by the way in which we confine the
electrons on the surface (thin-slab limit or zero thickness from the outset).
Another curiosity is that different minimal surfaces connected by the Bonnet
transformation (such as Schwarz's P- and D-surfaces) possess one-to-one
correspondence in their band energies at Brillouin zone boundaries.Comment: 6 pages, 8 figures, eps files will be sent on request to
[email protected]
Towards gravitationally assisted negative refraction of light by vacuum
Propagation of electromagnetic plane waves in some directions in
gravitationally affected vacuum over limited ranges of spacetime can be such
that the phase velocity vector casts a negative projection on the time-averaged
Poynting vector. This conclusion suggests, inter alia, gravitationally assisted
negative refraction by vacuum.Comment: 6 page
The structure of an LIM-only protein 4 (LMO4) and deformed epidermal autoregulatory factor-1 (DEAF1) complex reveals a common mode of binding to LMO4
LIM-domain only protein 4 (LMO4) is a widely expressed protein with important roles in embryonic development and breast cancer. It has been reported to bind many partners, including the transcription factor Deformed epidermal autoregulatory factor-1 (DEAF1), with which LMO4 shares many biological parallels. We used yeast two-hybrid assays to show that DEAF1 binds both LIM domains of LMO4 and that DEAF1 binds the same face on LMO4 as two other LMO4-binding partners, namely LIM domain binding protein 1 (LDB1) and C-terminal binding protein interacting protein (CtIP/RBBP8). Mutagenic screening analysed by the same method, indicates that the key residues in the interaction lie in LMO4LIM2 and the N-terminal half of the LMO4-binding domain in DEAF1. We generated a stable LMO4LIM2-DEAF1 complex and determined the solution structure of that complex. Although the LMO4-binding domain from DEAF1 is intrinsically disordered, it becomes structured on binding. The structure confirms that LDB1, CtIP and DEAF1 all bind to the same face on LMO4. LMO4 appears to form a hub in protein-protein interaction networks, linking numerous pathways within cells. Competitive binding for LMO4 therefore most likely provides a level of regulation between those different pathways.SJ was funded by an Australian Postgraduate Award (education.gov.au/australian-postgraduate-awards). JPM and JMM were awarded Senior Research
Fellowships from the Australian National and Medical Research Council (www.nhmrc.gov.au). This project was funded by an Australian Research Council (www.
arc.gov.au) Discovery Project Grant (DP110104332) to JMM and LC
A Renormalization Group for Hamiltonians: Numerical Results
We describe a renormalization group transformation that is related to the
breakup of golden invariant tori in Hamiltonian systems with two degrees of
freedom. This transformation applies to a large class of Hamiltonians, is
conceptually simple, and allows for accurate numerical computations. In a
numerical implementation, we find a nontrivial fixed point and determine the
corresponding critical index and scaling. Our computed values for various
universal constants are in good agreement with existing data for
area-preserving maps. We also discuss the flow associated with the nontrivial
fixed point.Comment: 11 Pages, 2 Figures. For future updates, check
ftp://ftp.ma.utexas.edu/pub/papers/koch
Serine, but not glycine, supports one-carbon metabolism and proliferation of cancer cells
Previous work has shown that some cancer cells are highly dependent on serine/glycine uptake for proliferation. Although serine and glycine can be interconverted and either might be used for nucleotide synthesis and one-carbon metabolism, we show that exogenous glycine cannot replace serine to support cancer cell proliferation. Cancer cells selectively consumed exogenous serine, which was converted to intracellular glycine and one-carbon units for building nucleotides. Restriction of exogenous glycine or depletion of the glycine cleavage system did not impede proliferation. In the absence of serine, uptake of exogenous glycine was unable to support nucleotide synthesis. Indeed, higher concentrations of glycine inhibited proliferation. Under these conditions, glycine was converted to serine, a reaction that would deplete the one-carbon pool. Providing one-carbon units by adding formate rescued nucleotide synthesis and growth of glycine-fed cells. We conclude that nucleotide synthesis and cancer cell proliferation are supported by serine—rather than glycine—consumption
Fast and flexible selection with a single switch
Selection methods that require only a single-switch input, such as a button
click or blink, are potentially useful for individuals with motor impairments,
mobile technology users, and individuals wishing to transmit information
securely. We present a single-switch selection method, "Nomon," that is general
and efficient. Existing single-switch selection methods require selectable
options to be arranged in ways that limit potential applications. By contrast,
traditional operating systems, web browsers, and free-form applications (such
as drawing) place options at arbitrary points on the screen. Nomon, however,
has the flexibility to select any point on a screen. Nomon adapts automatically
to an individual's clicking ability; it allows a person who clicks precisely to
make a selection quickly and allows a person who clicks imprecisely more time
to make a selection without error. Nomon reaps gains in information rate by
allowing the specification of beliefs (priors) about option selection
probabilities and by avoiding tree-based selection schemes in favor of direct
(posterior) inference. We have developed both a Nomon-based writing application
and a drawing application. To evaluate Nomon's performance, we compared the
writing application with a popular existing method for single-switch writing
(row-column scanning). Novice users wrote 35% faster with the Nomon interface
than with the scanning interface. An experienced user (author TB, with > 10
hours practice) wrote at speeds of 9.3 words per minute with Nomon, using 1.2
clicks per character and making no errors in the final text.Comment: 14 pages, 5 figures, 1 table, presented at NIPS 2009 Mini-symposi
Kramers-Kronig, Bode, and the meaning of zero
The implications of causality, as captured by the Kramers-Kronig relations
between the real and imaginary parts of a linear response function, are
familiar parts of the physics curriculum. In 1937, Bode derived a similar
relation between the magnitude (response gain) and phase. Although the
Kramers-Kronig relations are an equality, Bode's relation is effectively an
inequality. This perhaps-surprising difference is explained using elementary
examples and ultimately traces back to delays in the flow of information within
the system formed by the physical object and measurement apparatus.Comment: 8 pages; American Journal of Physics, to appea
Bayesian stochastic blockmodeling
This chapter provides a self-contained introduction to the use of Bayesian
inference to extract large-scale modular structures from network data, based on
the stochastic blockmodel (SBM), as well as its degree-corrected and
overlapping generalizations. We focus on nonparametric formulations that allow
their inference in a manner that prevents overfitting, and enables model
selection. We discuss aspects of the choice of priors, in particular how to
avoid underfitting via increased Bayesian hierarchies, and we contrast the task
of sampling network partitions from the posterior distribution with finding the
single point estimate that maximizes it, while describing efficient algorithms
to perform either one. We also show how inferring the SBM can be used to
predict missing and spurious links, and shed light on the fundamental
limitations of the detectability of modular structures in networks.Comment: 44 pages, 16 figures. Code is freely available as part of graph-tool
at https://graph-tool.skewed.de . See also the HOWTO at
https://graph-tool.skewed.de/static/doc/demos/inference/inference.htm
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