1,644 research outputs found
Discriminants, symmetrized graph monomials, and sums of squares
Motivated by the necessities of the invariant theory of binary forms J. J.
Sylvester constructed in 1878 for each graph with possible multiple edges but
without loops its symmetrized graph monomial which is a polynomial in the
vertex labels of the original graph. In the 20-th century this construction was
studied by several authors. We pose the question for which graphs this
polynomial is a non-negative resp. a sum of squares. This problem is motivated
by a recent conjecture of F. Sottile and E. Mukhin on discriminant of the
derivative of a univariate polynomial, and an interesting example of P. and A.
Lax of a graph with 4 edges whose symmetrized graph monomial is non-negative
but not a sum of squares. We present detailed information about symmetrized
graph monomials for graphs with four and six edges, obtained by computer
calculations
The existence of a real pole-free solution of the fourth order analogue of the Painleve I equation
We establish the existence of a real solution y(x,T) with no poles on the
real line of the following fourth order analogue of the Painleve I equation,
x=Ty-({1/6}y^3+{1/24}(y_x^2+2yy_{xx})+{1/240}y_{xxxx}). This proves the
existence part of a conjecture posed by Dubrovin. We obtain our result by
proving the solvability of an associated Riemann-Hilbert problem through the
approach of a vanishing lemma. In addition, by applying the Deift/Zhou
steepest-descent method to this Riemann-Hilbert problem, we obtain the
asymptotics for y(x,T) as x\to\pm\infty.Comment: 27 pages, 5 figure
A hadron model with breaking of spatial homogeneity of vacuum
A possible breaking of spatial homogeneity of vacuum due to the interaction
between quark and Bose-field is analyzed. It is shown that in this case quark
can be in a localized state (like wave packet). Energetic conditions for such a
spontaneous symmetry breaking are found in suggested model. Possible
consequences of such symmetry breaking, in particular, the origin of deep
inelastic processes and quark confinement phenomenon are discussed.Comment: 4 page
Response of a particle in a one-dimensional lattice to an applied force: Dynamics of the effective mass
We study the behaviour of the expectation value of the acceleration of a
particle in a one-dimensional periodic potential when an external homogeneous
force is suddenly applied. The theory is formulated in terms of modified Bloch
states that include the interband mixing induced by the force. This approach
allows us to understand the behaviour of the wavepacket, which responds with a
mass that is initially the bare mass, and subsequently oscillates around the
value predicted by the effective mass. If Zener tunneling can be neglected, the
expression obtained for the acceleration of the particle is valid over
timescales of the order of a Bloch oscillation, which are of interest for
experiments with cold atoms in optical lattices. We discuss how these
oscillations can be tuned in an optical lattice for experimental detection.Comment: 15 pages, 12 figure
Lax-Phillips scattering theory for PT-symmetric \rho-perturbed operators
The S-matrices corresponding to PT-symmetric \rho-perturbed operators are
defined and calculated by means of an approach based on an operator-theoretical
interpretation of the Lax-Phillips scattering theory
Magnetization pinning in conducting films demonstrated using broadband ferromagnetic resonance
The broadband microstrip ferromagnetic resonance technique has been applied
for detection and characterization of a magnetic inhomogeneity in a film
sample. In the case of a 100nm thick Permalloy film an additional magnetically
depleted top sub-layer, practically unidentifiable by the conventional
ferromagnetic resonance setup, has been detected and characterized. These
results have been confirmed by Brillouin light scattering spectroscopy
revealing the fact that the optical properties of the additional sub-layer do
not differ much from those of the bulk of the film. Subsequent characterization
of a large number of other presumably single-layer films with thicknesses in
the range 30-100nm using the same ferromagnetic resonance technique also
revealed the same effect
On asymptotic stability of the Skyrmion
We study the asymptotic behavior of spherically symmetric solutions in the
Skyrme model. We show that the relaxation to the degree-one soliton (called the
Skyrmion) has a universal form of a superposition of two effects: exponentially
damped oscillations (the quasinormal ringing) and a power law decay (the tail).
The quasinormal ringing, which dominates the dynamics for intermediate times,
is a linear resonance effect. In contrast, the polynomial tail, which becomes
uncovered at late times, is shown to be a \emph{nonlinear} phenomenon.Comment: 4 pages, 4 figures, minor changes to match the PRD versio
Astrocytes and Müller Cell Alterations During Retinal Degeneration in a Transgenic Rat Model of Retinitis Pigmentosa
Purpose: Retinitis pigmentosa includes a group of progressive retinal degenerative diseases that affect the structure and function of photoreceptors. Secondarily to the loss of photoreceptors, there is a reduction in retinal vascularization, which seems to influence the cellular degenerative process. Retinal macroglial cells, astrocytes, and Müller cells provide support for retinal neurons and are fundamental for maintaining normal retinal function. The aim of this study was to investigate the evolution of macroglial changes during retinal degeneration in P23H rats.
Methods: Homozygous P23H line-3 rats aged from P18 to 18 months were used to study the evolution of the disease, and SD rats were used as controls. Immunolabeling with antibodies against GFAP, vimentin, and transducin were used to visualize macroglial cells and cone photoreceptors.
Results: In P23H rats, increased GFAP labeling in Müller cells was observed as an early indicator of retinal gliosis. At 4 and 12 months of age, the apical processes of Müller cells in P23H rats clustered in firework-like structures, which were associated with ring-like shaped areas of cone degeneration in the outer nuclear layer. These structures were not observed at 16 months of age. The number of astrocytes was higher in P23H rats than in the SD matched controls at 4 and 12 months of age, supporting the idea of astrocyte proliferation. As the disease progressed, astrocytes exhibited a deteriorated morphology and marked hypertrophy. The increase in the complexity of the astrocytic processes correlated with greater connexin 43 expression and higher density of connexin 43 immunoreactive puncta within the ganglion cell layer (GCL) of P23H vs. SD rat retinas.
Conclusions: In the P23H rat model of retinitis pigmentosa, the loss of photoreceptors triggers major changes in the number and morphology of glial cells affecting the inner retina
Factorization of Seiberg-Witten Curves and Compactification to Three Dimensions
We continue our study of nonperturbative superpotentials of four-dimensional
N=2 supersymmetric gauge theories with gauge group U(N) on R^3 x S^1, broken to
N=1 due to a classical superpotential. In a previous paper, hep-th/0304061, we
discussed how the low-energy quantum superpotential can be obtained by
substituting the Lax matrix of the underlying integrable system directly into
the classical superpotential. In this paper we prove algebraically that this
recipe yields the correct factorization of the Seiberg-Witten curves, which is
an important check of the conjecture. We will also give an independent proof
using the algebraic-geometrical interpretation of the underlying integrable
system.Comment: laTeX, 14 pages, uses AMSmat
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