485 research outputs found
Galactic Punctuated Equilibrium: How to Undermine Carter's Anthropic Argument in Astrobiology
We investigate a new strategy which can defeat the (in)famous Carter's
"anthropic" argument against extraterrestrial life and intelligence. In
contrast to those already considered by Wilson, Livio, and others, the present
approach is based on relaxing hidden uniformitarian assumptions, considering
instead a dynamical succession of evolutionary regimes governed by both global
(Galaxy-wide) and local (planet- or planetary system-limited) regulation
mechanisms. This is in accordance with recent developments in both astrophysics
and evolutionary biology. Notably, our increased understanding of the nature of
supernovae and gamma-ray bursts, as well as of strong coupling between the
Solar System and the Galaxy on one hand, and the theories of "punctuated
equilibria" of Eldredge and Gould and "macroevolutionary regimes" of Jablonski,
Valentine, et al. on the other, are in full accordance with the regulation-
mechanism picture. The application of this particular strategy highlights the
limits of application of Carter's argument, and indicates that in the real
universe its applicability conditions are not satisfied. We conclude that
drawing far-reaching conclusions about the scarcity of extraterrestrial
intelligence and the prospects of our efforts to detect it on the basis of this
argument is unwarranted.Comment: 3 figures, 26 page
Elementary amenable subgroups of R. Thompson's group F
The subgroup structure of Thompson's group F is not yet fully understood. The
group F is a subgroup of the group PL(I) of orientation preserving, piecewise
linear self homeomorphisms of the unit interval and this larger group thus also
has a poorly understood subgroup structure. It is reasonable to guess that F is
the "only" subgroup of PL(I) that is not elementary amenable. In this paper, we
explore the complexity of the elementary amenable subgroups of F in an attempt
to understand the boundary between the elementary amenable subgroups and the
non-elementary amenable. We construct an example of an elementary amenable
subgroup up to class (height) omega squared, where omega is the first infinite
ordinal.Comment: 20 page
Classical and quantum ergodicity on orbifolds
We extend to orbifolds classical results on quantum ergodicity due to
Shnirelman, Colin de Verdi\`ere and Zelditch, proving that, for any positive,
first-order self-adjoint elliptic pseudodifferential operator P on a compact
orbifold X with positive principal symbol p, ergodicity of the Hamiltonian flow
of p implies quantum ergodicity for the operator P. We also prove ergodicity of
the geodesic flow on a compact Riemannian orbifold of negative sectional
curvature.Comment: 14 page
Distributed Generation and Resilience in Power Grids
We study the effects of the allocation of distributed generation on the
resilience of power grids. We find that an unconstrained allocation and growth
of the distributed generation can drive a power grid beyond its design
parameters. In order to overcome such a problem, we propose a topological
algorithm derived from the field of Complex Networks to allocate distributed
generation sources in an existing power grid.Comment: proceedings of Critis 2012 http://critis12.hig.no
Isomorphisms of Brin-Higman-Thompson groups
Let be positive integers with . Let
denote the ring that is universal with an invertible matrix. Let
denote the ring of matrices over the tensor
product of copies of . In a natural way, is a
partially ordered ring with involution. Let denote the
group of positive unitary elements. We show that is
isomorphic to the Brin-Higman-Thompson group ; the case was
found by Pardo, that is, is isomorphic to the Higman-Thompson group
. We survey arguments of Abrams, \'Anh, Bleak, Brin, Higman, Lanoue,
Pardo, and Thompson that prove that if and only
if , and (if and only if
and are isomorphic as
partially ordered rings with involution).Comment: 24 page
Quantum cat maps with spin 1/2
We derive a semiclassical trace formula for quantized chaotic transformations
of the torus coupled to a two-spinor precessing in a magnetic field. The trace
formula is applied to semiclassical correlation densities of the quantum map,
which, according to the conjecture of Bohigas, Giannoni and Schmit, are
expected to converge to those of the circular symplectic ensemble (CSE) of
random matrices. In particular, we show that the diagonal approximation of the
spectral form factor for small arguments agrees with the CSE prediction. The
results are confirmed by numerical investigations.Comment: 26 pages, 3 figure
Decay of correlations for maps with uniformly contracting fibers and logarithm law for singular hyperbolic attractors
We consider two dimensional maps preserving a foliation which is uniformly
contracting and a one dimensional associated quotient map having exponential
convergence to equilibrium (iterates of Lebesgue measure converge exponentially
fast to physical measure). We prove that these maps have exponential decay of
correlations over a large class of observables. We use this result to deduce
exponential decay of correlations for the Poincare maps of a large class of
singular hyperbolic flows. From this we deduce logarithm laws for these flows.Comment: 39 pages; 03 figures; proof of Theorem 1 corrected; many typos
corrected; improvements on the statements and comments suggested by a
referee. Keywords: singular flows, singular-hyperbolic attractor, exponential
decay of correlations, exact dimensionality, logarithm la
On dynamic network entropy in cancer
The cellular phenotype is described by a complex network of molecular
interactions. Elucidating network properties that distinguish disease from the
healthy cellular state is therefore of critical importance for gaining
systems-level insights into disease mechanisms and ultimately for developing
improved therapies. By integrating gene expression data with a protein
interaction network to induce a stochastic dynamics on the network, we here
demonstrate that cancer cells are characterised by an increase in the dynamic
network entropy, compared to cells of normal physiology. Using a fundamental
relation between the macroscopic resilience of a dynamical system and the
uncertainty (entropy) in the underlying microscopic processes, we argue that
cancer cells will be more robust to random gene perturbations. In addition, we
formally demonstrate that gene expression differences between normal and cancer
tissue are anticorrelated with local dynamic entropy changes, thus providing a
systemic link between gene expression changes at the nodes and their local
network dynamics. In particular, we also find that genes which drive
cell-proliferation in cancer cells and which often encode oncogenes are
associated with reductions in the dynamic network entropy. In summary, our
results support the view that the observed increased robustness of cancer cells
to perturbation and therapy may be due to an increase in the dynamic network
entropy that allows cells to adapt to the new cellular stresses. Conversely,
genes that exhibit local flux entropy decreases in cancer may render cancer
cells more susceptible to targeted intervention and may therefore represent
promising drug targets.Comment: 10 pages, 3 figures, 4 tables. Submitte
Genomic and Evolutionary Features of the SPI-1 Type III Secretion System That Is Present in Xanthomonas albilineans but Is Not Essential for Xylem Colonization and Symptom Development of Sugarcane Leaf Scald
Xanthomonas albilineans is the causal agent of sugarcane leaf scald. Interestingly, this bacterium, which is not known to be insect or animal associated, possesses a type III secretion system (T3SS) belonging to the injectisome family Salmonella pathogenicity island 1 (SPI-1). The T3SS SPI-1 of X. albilineans shares only low similarity with other available T3SS SPI-1 sequences. Screening of a collection of 128 plant-pathogenic bacteria revealed that this T3SS SPI-1 is present in only two species of Xanthomonas: X. albilineans and X. axonopodis pv. phaseoli. Inoculation of sugarcane with knockout mutants showed that this system is not required by X. albilineans to spread within xylem vessels and to cause disease symptoms. This result was confirmed by the absence of this T3SS SPI-1 in an X. albilineans strain isolated from diseased sugarcane. To investigate the importance of the T3SS SPI-1 during the life cycle of X. albilineans, we analyzed T3SS SPI-1 sequences from 11 strains spanning the genetic diversity of this species. No nonsense mutations or frameshifting indels were observed in any of these strains, suggesting that the T3SS SPI-1 system is maintained within the species X. albilineans. Evolutionary features of T3SS SPI-1 based on phylogenetic, recombination, and selection analyses are discussed in the context of the possible functional importance of T3SS SPI-1 in the ecology of X. albilineans
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