Matrix elements with respect to a discrete ‘‘Sturmian’’ basis set of the operator r1/2gr1/2, where g is the Coulomb Green’s function of the ‘‘second‐order’’ Dirac equation, are investigated. Closed expressions, involving Gaussian hypergeometric functions, can be obtained for the matrix elements. This calculation illustrates contour integration techniques capable of yielding closed expressions for a number of overlap integrals associated with the relativistic Kepler problem.

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