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254. GAMMAF: Subroutine to Compute the Incomplete
Gamma Function
by W. E. Baylis, Department of Physics, University of Windsor, Windsor, Ontario, Canada This function subprogram computes the incomplete gamma function: for any x > 0. Incomplete gamma functions are frequently encountered, for example, in the calculation of matrix elements of r in a Slater-function or Bates-Damgaard-function basis. For X < 2, power series expansions are used, whereas for x > 2, GAMMAF is evaluated by continued fractions (see M. Abramowitz and I. A. Stegun, Handbook of Mathematical Functions, NBS Appl. Math. Series 55, Washington, D. C. 1964, especially eq. (6.5.31)). Some combinations of A and X can make GAMMAF (A,X) overflow, for example, for A > 60 > X or for A < 0 and XA > 1080. FORTRAN IV Lines of Code: 108 Recommended Citation: W. E. Baylis, QCPE 11, 254 (1972). |