9,871 research outputs found
Renormalization Group results for lattice surface models
We study the phase diagram of statistical systems of closed and open
interfaces built on a cubic lattice. Interacting closed interfaces can be
written as Ising models, while open surfaces as Z(2) gauge systems. When the
open surfaces reduce to closed interfaces with few defects, also the gauge
model can be written as an Ising spin model. We apply the lower bound
renormalization group (LBRG) transformation introduced by Kadanoff (Phys. Rev.
Lett. 34, 1005 (1975)) to study the Ising models describing closed and open
surfaces with few defects. In particular, we have studied the Ising-like
transition of self-avoiding surfaces between the random-isotropic phase and the
phase with broken global symmetry at varying values of the mean curvature. Our
results are compared with previous numerical work. The limits of the LBRG
transformation in describing regions of the phase diagram where not
ferromagnetic ground-states are relevant are also discussed.Comment: 24 pages, latex, 5 figures (available upon request to
[email protected]
Pricing Step Options under the CEV and other Solvable Diffusion Models
We consider a special family of occupation-time derivatives, namely
proportional step options introduced by Linetsky in [Math. Finance, 9, 55--96
(1999)]. We develop new closed-form spectral expansions for pricing such
options under a class of nonlinear volatility diffusion processes which
includes the constant-elasticity-of-variance (CEV) model as an example. In
particular, we derive a general analytically exact expression for the resolvent
kernel (i.e. Green's function) of such processes with killing at an exponential
stopping time (independent of the process) of occupation above or below a fixed
level. Moreover, we succeed in Laplace inverting the resolvent kernel and
thereby derive newly closed-form spectral expansion formulae for the transition
probability density of such processes with killing. The spectral expansion
formulae are rapidly convergent and easy-to-implement as they are based simply
on knowledge of a pair of fundamental solutions for an underlying solvable
diffusion process. We apply the spectral expansion formulae to the pricing of
proportional step options for four specific families of solvable nonlinear
diffusion asset price models that include the CEV diffusion model and three
other multi-parameter state-dependent local volatility confluent hypergeometric
diffusion processes.Comment: 30 pages, 16 figures, submitted to IJTA
Extended d_{x^2 - y^2}-wave superconductivity
Remarkable anisotropic structures have been recently observed in the order
parameter Delta_k of the underdoped superconductor Bi-2212. Such findings are
strongly suggestive of deviations from a simple d_{x^2 - y^2}-wave picture of
high-Tc superconductivity, i.e. Delta_k ~ cos (k_x) - cos (k_y). In particular,
flatter nodes in Delta_k are observed along the k_x = (+/-) k_y directions in
k-space, than within this simple model for a d-wave gap. We argue that
nonlinear corrections in the k-dependence of Delta_k near the nodes introduce
new energy scales, which would lead to deviations in the predicted power-law
asymptotic behaviour of several measurable quantities, at low or intermediate
temperatures. We evaluate such deviations, either analytically or numerically,
within the interlayer pair-tunneling model, and within yet another
phenomenological model for a d-wave order parameter. We find that such
deviations are expected to be of different sign in the two cases. Moreover, the
doping dependence of the flatness of the gap near the nodes is also
attributable to Fermi surface effects, in addition to possible screening
effects modifying the in-plane pairing kernel, as recently proposed.Comment: 7 pages, 4 embedded PostScript figures. Uses svjour, epsfig, amsmath,
amssymb, xspace. To be published in Eur. Phys. J.
Homogenization Model for Aberrant Crypt Foci
Several explanations can be found in the literature about the origin of
colorectal cancer. There is however some agreement on the fact that the
carcinogenic process is a result of several genetic mutations of normal cells.
The colon epithelium is characterized by millions of invaginations, very small
cavities, called crypts, where most of the cellular activity occurs. It is
consensual in the medical community, that a potential first manifestation of
the carcinogenic process, observed in conventional colonoscopy images, is the
appearance of Aberrant Crypt Foci (ACF). These are clusters of abnormal crypts,
morphologically characterized by an atypical behavior of the cells that
populate the crypts. In this work an homogenization model is proposed, for
representing the cellular dynamics in the colon epithelium. The goal is to
simulate and predict, in silico, the spread and evolution of ACF, as it can be
observed in colonoscopy images. By assuming that the colon is an heterogeneous
media, exhibiting a periodic distribution of crypts, we start this work by
describing a periodic model, that represents the ACF cell-dynamics in a
two-dimensional setting. Then, homogenization techniques are applied to this
periodic model, to find a simpler model, whose solution symbolizes the averaged
behavior of ACF at the tissue level. Some theoretical results concerning the
existence of solution of the homogenized model are proven, applying a fixed
point theorem. Numerical results showing the convergence of the periodic model
to the homogenized model are presented.Comment: 26 pages, 4 figure
Evidence from multivariate morphometric study of the quercus pubescens complex in southeast Italy
The name Quercus pubescens s.l. encompasses a complex of deciduous oak taxa with mainly southeastEuropean
distribution and a large ecological niche. As the easternmost region of Italy, Apulia is
rather isolated from a geographical and physiographical viewpoint and counts the highest number
of oak species (10). In the taxonomic and phytosociological literature, the occurrence of several
species belonging to the Quercus pubescens collective group is reported for this region. In order to
verify if different sets of morphological characters are associated with different taxa, 24 populations
of Quercus pubescens s.l. located in different ecological-geographical areas of Apulia were sampled.
A total of 367 trees, 4254 leaves and 1120 fruits were collected and morphologically analysed.
Overall, 25 morphological characters of oak leaves and fruits were statistically treated using both
univariate and multivariate analysis. Nested ANOVA showed that leaves collected from a single tree
exhibited a degree of morphological variability higher than that observed when comparing leaves
coming from different trees of the same population and from different trees of different populations
as well. Almost all the morphological characters analysed exhibited a continuous trend of variation
so that none of them can be used as a character to discriminate between populations. Only leaf
and fruit “size” and fruit petiole length emerged as slightly discriminating characters. Our results
suggest that it is unlikely that more than one species belonging to the Quercus pubescens complex
occurs in the Apulia region. Comparison between the Apulian populations and a genetically
pure Q. pubescens population coming from a different area (the Molise region) strengthened the
assumption as to the existence of a single species that can provisionally be classified under the name
of Q. pubescens s.
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