The vicinity of an R-matrix pole

Abstract
Expressions for quantities such as reactance and scattering matrices, in terms of the R matrix, appear to become indeterminate at energies E equal to energies Ek at which the R matrix has poles. It is shown that determinate expressions are obtained using a diagonalisation procedure. Use of the procedure can give improved accuracy and stability in making numerical calculations at energies E such that mod E-Ek mod is small. Use of the method for bound-state calculations is also discussed.
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