Ambiguities in the Debye expansion of the elastic S matrix.

The possibility of expanding the S matrix in a multiple-reflection series (Debye expansion) is shown for an arbitrary-shape short-range potential. Ambiguity in the definition of outgoing and incoming waves inside the potential leads to an infinity of expansions. These are analyzed in a simple example (square well or barrier) in order to characterize the correct choice of outgoing and incoming waves. [ABSTRACT FROM AUTHOR]