Annals of Combinatorics 4 (2000) 307-316
Graph and Group Classifications of Terms in Adiabatic and Semiclassical Expansions
Mathematics Department, Texas A&M University, College Station, TX
Received April 14, 1999
AMS Subject Classification: 05C30, 20C35, 81Q20
Abstract. The author describe three ways in which concepts from discrete mathematics are being used, along with computer algebra systems, to bring order into the welter of terms that appear when asymptotic expansions of propagators or heat kernels are carried out to high orders. (The key needs are (a) specifying a linearly independent set of terms, and (b) classifying terms by their structural properties.) (1) For a Schrödinger operator with a scalar potential, a closed-form solution has been given as a sum over graphs (including loops and multiple lines). To generate the series requires only the listing of all graphs with given numbers of lines and vertices, taking account of isomorphism. This can be done by Pólya-type methods, obtaining along the way a finer classification of graphs by the distribution of lines among the vertices. (2) For problems involving a Riemann curvature tensor, enumerating and classifying all the linearly independent terms of each order involves the Schur-Young-Littlewood theory of representations of the symmetric and orthogonal groups. (3) For scalar equations, the connectedness of graphs has a profound relation to the mathematical structure of the series. If there is an analogue of this principle for equations with matrix-valued potentials, the relevant notion of connectedness must involve commutators as well as index contractions.
Keywords: asymptotic expansion, semiclassical approximation, tensor, multigraph, propagator, group representation
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