81 research outputs found
Fluctuation symmetries for work and heat
We consider a particle dragged through a medium at constant temperature as
described by a Langevin equation with a time-dependent potential. The
time-dependence is specified by an external protocol. We give conditions on
potential and protocol under which the dissipative work satisfies an exact
symmetry in its fluctuations for all times. We also present counter examples to
that exact fluctuation symmetry when our conditions are not satisfied. Finally,
we consider the dissipated heat which differs from the work by a temporal
boundary term. We explain when and why there can be a correction to the
standard fluctuation theorem due to the unboundedness of that temporal
boundary. However, the corrected fluctuation symmetry has again a general
validity.Comment: 10 pages, 4 figures (v2: minor typographic corrections
Vanna-Volga methods applied to FX derivatives : from theory to market practice
We study Vanna-Volga methods which are used to price first generation exotic
options in the Foreign Exchange market. They are based on a rescaling of the
correction to the Black-Scholes price through the so-called `probability of
survival' and the `expected first exit time'. Since the methods rely heavily on
the appropriate treatment of market data we also provide a summary of the
relevant conventions. We offer a justification of the core technique for the
case of vanilla options and show how to adapt it to the pricing of exotic
options. Our results are compared to a large collection of indicative market
prices and to more sophisticated models. Finally we propose a simple
calibration method based on one-touch prices that allows the Vanna-Volga
results to be in line with our pool of market data
Attractor Modulation and Proliferation in 1+ Dimensional Neural Networks
We extend a recently introduced class of exactly solvable models for
recurrent neural networks with competition between 1D nearest neighbour and
infinite range information processing. We increase the potential for further
frustration and competition in these models, as well as their biological
relevance, by adding next-nearest neighbour couplings, and we allow for
modulation of the attractors so that we can interpolate continuously between
situations with different numbers of stored patterns. Our models are solved by
combining mean field and random field techniques. They exhibit increasingly
complex phase diagrams with novel phases, separated by multiple first- and
second order transitions (dynamical and thermodynamic ones), and, upon
modulating the attractor strengths, non-trivial scenarios of phase diagram
deformation. Our predictions are in excellent agreement with numerical
simulations.Comment: 16 pages, 15 postscript figures, Late
Slowly evolving random graphs II: Adaptive geometry in finite-connectivity Hopfield models
We present an analytically solvable random graph model in which the
connections between the nodes can evolve in time, adiabatically slowly compared
to the dynamics of the nodes. We apply the formalism to finite connectivity
attractor neural network (Hopfield) models and we show that due to the
minimisation of the frustration effects the retrieval region of the phase
diagram can be significantly enlarged. Moreover, the fraction of misaligned
spins is reduced by this effect, and is smaller than in the infinite
connectivity regime. The main cause of this difference is found to be the
non-zero fraction of sites with vanishing local field when the connectivity is
finite.Comment: 17 pages, 8 figure
Statics and dynamics of the Lebwohl-Lasher model in the Bethe approximation
We study the Lebwohl-Lasher model for systems in which spin are arranged on
random graph lattices. At equilibrium our analysis follows the theory of
spin-systems on random graphs which allows us to derive exact bifurcation
conditions for the phase diagram. We also study the dynamics of this model
using a variant of the dynamical replica theory. Our results are tested against
simulations.Comment: 16 pages, 5 eps figures, elsart; extended result
Dynamic rewiring in small world networks
We investigate equilibrium properties of small world networks, in which both
connectivity and spin variables are dynamic, using replicated transfer matrices
within the replica symmetric approximation. Population dynamics techniques
allow us to examine order parameters of our system at total equilibrium,
probing both spin- and graph-statistics. Of these, interestingly, the degree
distribution is found to acquire a Poisson-like form (both within and outside
the ordered phase). Comparison with Glauber simulations confirms our results
satisfactorily.Comment: 21 pages, 5 figure
Analysis of common attacks in LDPCC-based public-key cryptosystems
We analyze the security and reliability of a recently proposed class of
public-key cryptosystems against attacks by unauthorized parties who have
acquired partial knowledge of one or more of the private key components and/or
of the plaintext. Phase diagrams are presented, showing critical partial
knowledge levels required for unauthorized decryptionComment: 14 pages, 6 figure
Synchronous versus sequential updating in the three-state Ising neural network with variable dilution
The three-state Ising neural network with synchronous updating and variable
dilution is discussed starting from the appropriate Hamiltonians. The
thermodynamic and retrieval properties are examined using replica mean-field
theory. Capacity-temperature phase diagrams are derived for several values of
the pattern activity and different gradations of dilution, and the information
content is calculated. The results are compared with those for sequential
updating. The effect of self-coupling is established. Also the dynamics is
studied using the generating function technique for both synchronous and
sequential updating. Typical flow diagrams for the overlap order parameter are
presented. The differences with the signal-to-noise approach are outlined.Comment: 21 pages Latex, 12 eps figures and 1 ps figur
A Solvable Model of Secondary Structure Formation in Random Hetero-Polymers
We propose and solve a simple model describing secondary structure formation
in random hetero-polymers. It describes monomers with a combination of
one-dimensional short-range interactions (representing steric forces and
hydrogen bonds) and infinite range interactions (representing polarity forces).
We solve our model using a combination of mean field and random field
techniques, leading to phase diagrams exhibiting second-order transitions
between folded, partially folded and unfolded states, including regions where
folding depends on initial conditions. Our theoretical results, which are in
excellent agreement with numerical simulations, lead to an appealing physical
picture of the folding process: the polarity forces drive the transition to a
collapsed state, the steric forces introduce monomer specificity, and the
hydrogen bonds stabilise the conformation by damping the frustration-induced
multiplicity of states.Comment: 24 pages, 14 figure
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