9,377 research outputs found
Online Local Volatility Calibration by Convex Regularization with Morozov's Principle and Convergence Rates
We address the inverse problem of local volatility surface calibration from
market given option prices. We integrate the ever-increasing flow of option
price information into the well-accepted local volatility model of Dupire. This
leads to considering both the local volatility surfaces and their corresponding
prices as indexed by the observed underlying stock price as time goes by in
appropriate function spaces. The resulting parameter to data map is defined in
appropriate Bochner-Sobolev spaces. Under this framework, we prove key
regularity properties. This enable us to build a calibration technique that
combines online methods with convex Tikhonov regularization tools. Such
procedure is used to solve the inverse problem of local volatility
identification. As a result, we prove convergence rates with respect to noise
and a corresponding discrepancy-based choice for the regularization parameter.
We conclude by illustrating the theoretical results by means of numerical
tests.Comment: 23 pages, 5 figure
Moduli and K\"ahler potential in fermionic strings
We study the problem of identifying the moduli fields in fermionic
four-dimensional string models. We deform a free-fermionic model by introducing
exactly marginal operators in the form of Abelian Thirring interactions on the
world-sheet, and show that their couplings correspond to the untwisted moduli
fields. We study the consequences of this method for simple free-fermionic
models which correspond to orbifolds and obtain their moduli
space and K\"ahler potential by symmetry arguments and by direct calculation of
string scattering amplitudes. We then generalize our analysis to more
complicated fermionic structures which arise in constructions of realistic
models corresponding to asymmetric orbifolds, and obtain the moduli space and
K\"ahler potential for this case. Finally we extend our analysis to the
untwisted matter sector and derive expressions for the full K\"ahler potential
to be used in phenomenological applications, and the target space duality
transformations of the corresponding untwisted matter fields.Comment: 27pp Latex text, no figs, CERN-TH.7259/94, CTP-TAMU-14/94 and
ACT-06/9
Phase and Charge reentrant phase transitions in two capacitively coupled Josephson arrays with ultra-small junction
We have studied the phase diagram of two capacitively coupled Josephson
junction arrays with charging energy, , and Josephson coupling energy,
. Our results are obtained using a path integral Quantum Monte Carlo
algorithm. The parameter that quantifies the quantum fluctuations in the i-th
array is defined by . Depending on
the value of , each independent array may be in the semiclassical or
in the quantum regime: We find that thermal fluctuations are important when
and the quantum fluctuations dominate when . We have extensively studied the interplay between vortex and charge
dominated individual array phases. The two arrays are coupled via the
capacitance at each site of the lattices. We find a {\it
reentrant transition} in , at low temperatures, when one of
the arrays is in the semiclassical limit (i.e. ) and the
quantum array has , for the values considered for
the interlayer capacitance. In addition, when , and
for all the inter-layer couplings considered above, a {\it novel} reentrant
phase transition occurs in the charge degrees of freedom, i.e. there is a
reentrant insulating-conducting transition at low temperatures. We obtain the
corresponding phase diagrams and found some features that resemble those seen
in experiments with 2D JJA.Comment: 25 Latex pages including 8 encapsulated poscript figures. Accepted
for publication in Phys. Rev B (Nov. 2004 Issue
M-theory Inspired No-scale Supergravity
We propose a supergravity model that contains elements recently shown to
arise in the strongly-coupled limit of the heterotic string
(M-theory), including a no-scale--like K\"ahler potential, the identification
of the string scale with the gauge coupling unification scale, and the onset of
supersymmetry breaking at an intermediate scale determined by the size of the
eleventh dimension of M-theory. We also study the phenomenological consequences
of such scenario, which include a rather constrained sparticle spectrum within
the reach of present-generation particle accelerators.Comment: 8 pages, LaTeX, 3 figures (included
On the thermal and double episode emissions in GRB 970828
Following the recent theoretical interpretation of GRB 090618 and GRB 101023,
we here interpret GRB 970828 in terms of a double episode emission: the first
episode, observed in the first 40 s of the emission, is interpreted as the
proto-black-hole emission; the second episode, observed after t+50 s, as a
canonical gamma ray burst. The transition between the two episodes marks the
black hole formation. The characteristics of the real GRB, in the second
episode, are an energy of erg, a
baryon load of and a bulk Lorentz factor at transparency
of . The clear analogy with GRB 090618 would require also in
GRB 970828 the presence of a possible supernova. We also infer that the GRB
exploded in an environment with a large average particle density part/cm and dense clouds characterized by typical dimensions
of cm and . Such an environment
is in line with the observed large column density absorption, which might have
darkened both the supernova emission and the GRB optical afterglow.Comment: 7 pages, 10 figures, submitted to Ap
Recovered Memories, Extended Statutes of Limitations and Discovery Exceptions in Childhood Sexual Abuse Cases: Have We Gone Too Far?
Cosmological string models from Milne spaces and SL(2,Z) orbifold
The -dimensional Milne Universe with extra free directions is used to
construct simple FRW cosmological string models in four dimensions, describing
expansion in the presence of matter with , . We then
consider the n=2 case and make SL(2,Z) orbifold identifications. The model is
surprisingly related to the null orbifold with an extra reflection generator.
The study of the string spectrum involves the theory of harmonic functions in
the fundamental domain of SL(2,Z). In particular, from this theory one can
deduce a bound for the energy gap and the fact that there are an infinite
number of excitations with a finite degeneracy. We discuss the structure of
wave functions and give examples of physical winding states becoming light near
the singularity.Comment: 14 pages, harvma
Linear Theory of Electron-Plasma Waves at Arbitrary Collisionality
The dynamics of electron-plasma waves are described at arbitrary
collisionality by considering the full Coulomb collision operator. The
description is based on a Hermite-Laguerre decomposition of the velocity
dependence of the electron distribution function. The damping rate, frequency,
and eigenmode spectrum of electron-plasma waves are found as functions of the
collision frequency and wavelength. A comparison is made between the
collisionless Landau damping limit, the Lenard-Bernstein and Dougherty
collision operators, and the electron-ion collision operator, finding large
deviations in the damping rates and eigenmode spectra. A purely damped entropy
mode, characteristic of a plasma where pitch-angle scattering effects are
dominant with respect to collisionless effects, is shown to emerge numerically,
and its dispersion relation is analytically derived. It is shown that such a
mode is absent when simplified collision operators are used, and that
like-particle collisions strongly influence the damping rate of the entropy
mode.Comment: 23 pages, 10 figures, accepted for publication on Journal of Plasma
Physic
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