End-To-End Distribution Function Function of Stiff Polymers for all Persistence Lengths

We set up recursion relations for calculating all even moments of the end-to-end distance of a Porod-Kratky wormlike chains in $D$ dimensions. From these moments we derive a simple analytic expression for the end-to-end distribution in three dimensions valid for all peristence lengths. It is in excellent agreement with Monte Carlo data for stiff chains and goes properly over into the Gaussian random-walk distributions for low stiffness.Comment: Author Information under http://www.physik.fu-berlin.de/~kleinert/institution.html . Latest update of paper (including all PS fonts) at http://www.physik.fu-berlin.de/~kleinert/345 Mathematica programs at http://www.physik.fu-berlin.de/~kleinert/b5/pgm1

The Highest-Derivative Version of Variational Perturbation Theory

We systematically investigate different versions of variational perturbation theory by forcing not only the first or second but also higher derivatives of the approximant with respect to the variational parameter to vanish. The choice of the highest derivative version turns out to be the most successful one for approximating the ground-state energy of the anharmonic oscillator. It is therefore used to determine the critical exponent alpha of the specific heat in superfluid 4He in agreement with the value measured in recent space shuttle experiments.Comment: The article is a contribution to the book "Fluctuating Paths and Fields - Dedicated to Hagen Kleinert on the Occasion of His 60th Birthday", Eds. Wolfhard Janke, Axel Pelster, Hans-Juergen Schmidt, and Michael Bachmann (World Scientific, Singapore, 2001), p. 347-36

Variational Perturbation Theory for Summing Divergent Non-Borel-Summable Tunneling Amplitudes

We present a method for evaluating divergent non-Borel-summable series by an analytic continuation of variational perturbation theory. We demonstrate the power of the method by an application to the exactly known partition function of the anharmonic oscillator in zero spacetime dimensions. In one spacetime dimension we derive the imaginary part of the ground state energy of the anharmonic oscillator for {\em all negative values of the coupling constant $g$, including the nonanalytic tunneling regime at small-$g$. As a highlight of the theory we retrieve from the divergent perturbation expansion the action of the critical bubble and the contribution of the higher loop fluctuations around the bubble.Comment: Author Information under http://www.physik.fu-berlin.de/~kleinert/institution.html . Latest update of paper (including all PS fonts) at http://www.physik.fu-berlin.de/~kleinert/34

Perturbation Theory for Particle in a Box

Recently developed strong-coupling theory open up the possibility of treating quantum-mechanical systems with hard-wall potentials via perturbation theory. To test the power of this theory we study here the exactly solvable quantum mechanics of a point particle in a one-dimensional box. Introducing an auxiliary harmonic mass term $m$, the ground-state energy E^{(0) can be expanded perturbatively in powers of $1/md$, where $d$ is the box size. The removal of the infrared cutoff $m$ requires the resummation of the series at infinitely strong coupling. We show that strong-coupling theory yields a fast-convergent sequence of approximations to the well-known quantum-mechanical energy E^{(0)= \pi ^2/2d^2.Comment: Author Information under http://www.physik.fu-berlin.de/~kleinert/institution.html . Latest update of paper also at http://www.physik.fu-berlin.de/~kleinert/28

Runtime-Flexible Multi-dimensional Arrays and Views for C++98 and C++0x

Multi-dimensional arrays are among the most fundamental and most useful data structures of all. In C++, excellent template libraries exist for arrays whose dimension is fixed at runtime. Arrays whose dimension can change at runtime have been implemented in C. However, a generic object-oriented C++ implementation of runtime-flexible arrays has so far been missing. In this article, we discuss our new implementation called Marray, a package of class templates that fills this gap. Marray is based on views as an underlying concept. This concept brings some of the flexibility known from script languages such as R and MATLAB to C++. Marray is free both for commercial and non-commercial use and is publicly available from www.andres.sc/marrayComment: Free source code availabl