55,692 research outputs found
The phase transition in random catalytic sets
The notion of (auto) catalytic networks has become a cornerstone in
understanding the possibility of a sudden dramatic increase of diversity in
biological evolution as well as in the evolution of social and economical
systems. Here we study catalytic random networks with respect to the final
outcome diversity of products. We show that an analytical treatment of this
longstanding problem is possible by mapping the problem onto a set of
non-linear recurrence equations. The solution of these equations show a crucial
dependence of the final number of products on the initial number of products
and the density of catalytic production rules. For a fixed density of rules we
can demonstrate the existence of a phase transition from a practically
unpopulated regime to a fully populated and diverse one. The order parameter is
the number of final products. We are able to further understand the origin of
this phase transition as a crossover from one set of solutions from a quadratic
equation to the other.Comment: 7 pages, ugly eps files due to arxiv restriction
Solving Vlasov Equations Using NRxx Method
In this paper, we propose a moment method to numerically solve the Vlasov
equations using the framework of the NRxx method developed in [6, 8, 7] for the
Boltzmann equation. Due to the same convection term of the Boltzmann equation
and the Vlasov equation, it is very convenient to use the moment expansion in
the NRxx method to approximate the distribution function in the Vlasov
equations. The moment closure recently presented in [5] is applied to achieve
the globally hyperbolicity so that the local well-posedness of the moment
system is attained. This makes our simulations using high order moment
expansion accessible in the case of the distribution far away from the
equilibrium which appears very often in the solution of the Vlasov equations.
With the moment expansion of the distribution function, the acceleration in the
velocity space results in an ordinary differential system of the macroscopic
velocity, thus is easy to be handled. The numerical method we developed can
keep both the mass and the momentum conserved. We carry out the simulations of
both the Vlasov-Poisson equations and the Vlasov-Poisson-BGK equations to study
the linear Landau damping. The numerical convergence is exhibited in terms of
the moment number and the spatial grid size, respectively. The variation of
discretized energy as well as the dependence of the recurrence time on moment
order is investigated. The linear Landau damping is well captured for different
wave numbers and collision frequencies. We find that the Landau damping rate
linearly and monotonically converges in the spatial grid size. The results are
in perfect agreement with the theoretic data in the collisionless case
Algorithms for efficient vectorization of repeated sparse power system network computations
Cataloged from PDF version of article.Standard sparsity-based algorithms used in power system
appllcations need to be restructured for efficient vectorization
due to the extremely short vectors processed. Further, intrinsic
architectural features of vector computers such as chaining and
sectioning should also be exploited for utmost performance. This
paper presents novel data storage schemes and vectorization alsorim
that resolve the recurrence problem, exploit chaining and
minimize the number of indirect element selections in the repeated
solution of sparse linear system of equations widely encountered
in various power system problems. The proposed schemes are
also applied and experimented for the vectorization of power mismatch
calculations arising in the solution phase of FDLF which involves
typical repeated sparse power network computations. The
relative performances of the proposed and existing vectorization
schemes are evaluated, both theoretically and experimentally on
IBM 3090ArF.Standard sparsity-based algorithms used in power system appllcations need to be restructured for efficient vectorization
due to the extremely short vectors processed. Further, intrinsic architectural features of vector computers such as chaining and sectioning should also be exploited for utmost performance. This paper presents novel data storage schemes and vectorization alsorim that resolve the recurrence problem, exploit chaining and minimize the number of indirect element selections in the repeated solution of sparse linear system of equations widely encountered in various power system problems. The proposed schemes are also applied and experimented for the vectorization of power mismatch calculations arising in the solution phase of FDLF which involves typical repeated sparse power network computations. The relative performances of the proposed and existing vectorization schemes are evaluated, both theoretically and experimentally on IBM 3090ArF
Some investigations into the numerical solution of initial value problems in ordinary differential equations
PhD ThesisIn this thesis several topics in the numerical solution
of the initial value problem in first-order ordinary diff'erentlal
equations are investigated,
Consideration is given initially to stiff differential
equations and their solution by stiffly-stable linear multistep
methods which incorporate second derivative terms. Attempts are
made to increase the size of the stability regions for these
methods both by particular choices for the third characteristic
polynomial and by the use of optimization techniques while
investigations are carried out regarding the capabilities of a
high order method.
Subsequent work is concerned with the development of
Runge-Kutta methods which include second-derivative terms and
are implicit with respect to y rather than k. Methods of
order three and four are proposed which are L-stable.
The major part of the thesis is devoted to the establishment
of recurrence relations for operators associated with linear
multistep methods which are based on a non-polynomial
representation of the theoretical solution. A complete set of
recurrence relations is developed for both implicit and
explicit multistep methods which are based on a representation
involving a polynomial part and any number of arbitrary functions.
The amount of work involved in obtaining mulc iste, :ne::l'lJds by this
technique is considered and criteria are proposed to Jecide when
this particular method of derivation should be em~loyed.
The thesis is conclud~d by using Prony's method to develop
one-step methods and multistep methods which are exponentially
adaptive and as such can be useful in obtaining solutions to
problems which are exponential in nature
A toolbox to solve coupled systems of differential and difference equations
We present algorithms to solve coupled systems of linear differential
equations, arising in the calculation of massive Feynman diagrams with local
operator insertions at 3-loop order, which do {\it not} request special choices
of bases. Here we assume that the desired solution has a power series
representation and we seek for the coefficients in closed form. In particular,
if the coefficients depend on a small parameter \ep (the dimensional
parameter), we assume that the coefficients themselves can be expanded in
formal Laurent series w.r.t.\ \ep and we try to compute the first terms in
closed form. More precisely, we have a decision algorithm which solves the
following problem: if the terms can be represented by an indefinite nested
hypergeometric sum expression (covering as special cases the harmonic sums,
cyclotomic sums, generalized harmonic sums or nested binomial sums), then we
can calculate them. If the algorithm fails, we obtain a proof that the terms
cannot be represented by the class of indefinite nested hypergeometric sum
expressions. Internally, this problem is reduced by holonomic closure
properties to solving a coupled system of linear difference equations. The
underlying method in this setting relies on decoupling algorithms, difference
ring algorithms and recurrence solving. We demonstrate by a concrete example
how this algorithm can be applied with the new Mathematica package
\texttt{SolveCoupledSystem} which is based on the packages \texttt{Sigma},
\texttt{HarmonicSums} and \texttt{OreSys}. In all applications the
representation in -space is obtained as an iterated integral representation
over general alphabets, generalizing Poincar\'{e} iterated integrals
Formulas for Continued Fractions. An Automated Guess and Prove Approach
We describe a simple method that produces automatically closed forms for the
coefficients of continued fractions expansions of a large number of special
functions. The function is specified by a non-linear differential equation and
initial conditions. This is used to generate the first few coefficients and
from there a conjectured formula. This formula is then proved automatically
thanks to a linear recurrence satisfied by some remainder terms. Extensive
experiments show that this simple approach and its straightforward
generalization to difference and -difference equations capture a large part
of the formulas in the literature on continued fractions.Comment: Maple worksheet attache
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