6,810 research outputs found
Moduli Spaces and Formal Operads
Let overline{M}_{g,n} be the moduli space of stable algebraic curves of genus
g with n marked points. With the operations which relate the different moduli
spaces identifying marked points, the family (overline{M}_{g,n})_{g,n} is a
modular operad of projective smooth Deligne-Mumford stacks, overline{M}. In
this paper we prove that the modular operad of singular chains
C_*(overline{M};Q) is formal; so it is weakly equivalent to the modular operad
of its homology H_*(overline{M};Q). As a consequence, the "up to homotopy"
algebras of these two operads are the same. To obtain this result we prove a
formality theorem for operads analogous to Deligne-Griffiths-Morgan-Sullivan
formality theorem, the existence of minimal models of modular operads, and a
characterization of formality for operads which shows that formality is
independent of the ground field.Comment: 36 pages (v3: some typographical corrections
Distribución biogeográfica de las hormigas (Hymenoptera, Formicidae) en las Islas del Mediterráneo Occidental
Abstract not availabl
A Conformal Mapping and Isothermal Perfect Fluid Model
Instead of conformal to flat spacetime, we take the metric conformal to a
spacetime which can be thought of as ``minimally'' curved in the sense that
free particles experience no gravitational force yet it has non-zero curvature.
The base spacetime can be written in the Kerr-Schild form in spherical polar
coordinates. The conformal metric then admits the unique three parameter family
of perfect fluid solution which is static and inhomogeneous. The density and
pressure fall off in the curvature radial coordinates as for
unbounded cosmological model with a barotropic equation of state. This is the
characteristic of isothermal fluid. We thus have an ansatz for isothermal
perfect fluid model. The solution can also represent bounded fluid spheres.Comment: 10 pages, TeX versio
On the theory of diamagnetism in granular superconductors
We study a highly disordered network of superconducting granules linked by
weak Josephson junctions in magnetic field and develop a mean field theory for
this problem. The diamagnetic response to a slow {\it variations} of magnetic
field is found to be analogous to the response of a type-II superconductor with
extremely strong pinning. We calculate an effective penetration depth
and critical current and find that both and
are non-zero but are strongly suppressed by frustration.Comment: REVTEX, 12 pages, two Postscript figure
High-frequency effects in the FitzHugh-Nagumo neuron model
The effect of a high-frequency signal on the FitzHugh-Nagumo excitable model
is analyzed. We show that the firing rate is diminished as the ratio of the
high-frequency amplitude to its frequency is increased. Moreover, it is
demonstrated that the excitable character of the system, and consequently the
firing activity, is suppressed for ratios above a given threshold value. In
addition, we show that the vibrational resonance phenomenon turns up for
sufficiently large noise strength values.Comment: 4 pages, 4 figures (to appear in Physical Review E
Twist-3 distribution amplitudes of scalar mesons from QCD sum rules
We study the twist-3 distribution amplitudes for scalar mesons made up of two
valence quarks based on QCD sum rules.
By choosing the proper correlation functions, we derive the moments of the
scalar mesons up to the first two order. Making use of these moments, we then
calculate the first two Gegenbauer coefficients for twist-3 distribution
amplitudes of scalar mesons. It is found that the second Gegenbauer
coefficients of scalar density twist-3 distribution amplitudes for
and mesons are quite close to that for , which indicates that the
SU(3) symmetry breaking effect is tiny here. However, this effect could not be
neglected for the forth Gegenbauer coefficients of scalar twist-3 distribution
amplitudes between and . Besides, we also observe that the first two
Gegenbauer coefficients corresponding to the tensor current twist-3
distribution amplitudes for all the , and are very small.
The renormalization group evolution of condensates, quark masses, decay
constants and moments are considered in our calculations. As a byproduct, it is
found that the masses for isospin I=1, scalar mesons are around
GeV and GeV respectively, while the mass for
isospin state composed of is GeV.Comment: replaced with revised version, to be published in Phys. Rev.
Two-state theory of nonlinear Stochastic Resonance
An amenable, analytical two-state description of the nonlinear population
dynamics of a noisy bistable system driven by a rectangular subthreshold signal
is put forward. Explicit expressions for the driven population dynamics, the
correlation function (its coherent and incoherent part), the signal-to-noise
ratio (SNR) and the Stochastic Resonance (SR) gain are obtained. Within a
suitably chosen range of parameter values this reduced description yields
anomalous SR-gains exceeding unity and, simultaneously, gives rise to a
non-monotonic behavior of the SNR vs. the noise strength. The analytical
results agree well with those obtained from numerical solutions of the Langevin
equation.Comment: 4 pages, 1 figur
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