The programming assistance provided by A. Agay of the Hebrew University in expediting the numerical solution of the eigenvalue problem is greatly appreciated. Detailed comments of two anonymous reviewers helped clarify the presentation in several places. This study was funded by Office of Naval Research Grant N00014-93-1-0673 to UCLA and by an Israel Academy of Sciences grant to the Hebrew University of Jerusalem. MG is indebted to the Académie des Sciences, Paris, for its Elf-Aquitaine/C.N.R.S. Chair 1995–96, which facilitated the completion of the work. K. E. Hartman helped with the presentation.
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