  Closed formulas in terms of double sums of Clebsch-Gordan coefficients are
computed for the evaluation of bra-ket spherical harmonic overlap integrals of
a wide class of trigonometric functions. These analytical expressions can find
useful application in problems involving non-separable wave equations, e.g.
general-relativistic perturbation theory, electromagnetism, quantum theory,
etc, wherein the overlap integrals arise from the coupling among different
angular modes.
