Evaluation of the modified Bessel function of the third kind of imaginary orders

The evaluation of the modified Bessel function of the third kind of purely imaginary order K-ia(x) is discussed, we also present analogous results for the derivative. The methods are based on the use of Maclaurin series. nonoscillatory integral representations, asymptotic expansions, and a continued fraction method, depending on the ranges of x and a. We discuss the range of applicability of the different approaches considered and conclude that power series, the continued fraction method. and the nonoscillatory integral representation can be used to accurately compute the function K-ia(x) in the range 0 less than or equal to a less than or equal to 200, 0 less than or equal to x less than or equal to 100, using a similar scheme the derivative K'(ia) (x) can also be computed within these ranges. (C) 2002 Elsevier Science (USA).