Predrag Cvitanovic'
Trace formulas for stochastic evolution operators
 
Intuitively, the noise inherent in any realistic system washes out
fine details and makes chaotic averages more robust. Quantum mechanical
h-bar resolution of phase space implies that in semi-classical approaches
no orbits longer than the Heisenberg time need be taken into account.
We explore these ideas in some detail by casting stochastic dynamics
into path integral form and developing perturbative and nonperturbative
methods for evaluating such such integrals. In the weak noise case the
standard perturbation theory is expansion in terms of Feynman diagrams.
Now the surprise; we can compute the same corrections faster and to a 
higher order in perturbation theory by integrating over the neighborhood 
of a given saddlepoint exactly by means of a nonlinear change of
field variables.  The new perturbative expansion appears more compact
than the standard Feynman diagram perturbation theory; whether it is
better than traditional loop expansions for computing field-theoretic
saddlepoint expansions remains to be seen, but for a simple system we study
the result is a stochastic analog of the Gutzwiller trace formula
with  the $\hbar$ corrections so far computed to five orders higher than
what has been attainable in the quantum-mechanical applications.
 
U. Chicago - 
Computations in Science Seminars
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Feb. 10, 1999