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.