DGAMRN(3)

SYNOPSIS
DOUBLE PRECISION FUNCTION DGAMRN (X)

NAME
dgamrn - Subsidiary to DBSKIN

LIBRARY
SLATEC

TYPE
DOUBLE PRECISION (GAMRN-S, DGAMRN-D)

AUTHOR
Amos, D. E., (SNLA)

DESCRIPTION
Abstract * A Double Precision Routine * DGAMRN computes the GAMMA function ratio GAMMA(X)/GAMMA(X+0.5) for real X.gt.0. If X.ge.XMIN, an asymptotic expansion is evaluated. If X.lt.XMIN, an integer is added to X to form a new value of X.ge.XMIN and the asymptotic expansion is eval- uated for this new value of X. Successive application of the recurrence relation W(X)=W(X+1)*(1+0.5/X) reduces the argument to its original value. XMIN and comp- utational tolerances are computed as a function of the number of digits carried in a word by calls to I1MACH and D1MACH. However, the computational accuracy is limited to the max- imum of unit roundoff (=D1MACH(4)) and 1.0D-18 since critical constants are given to only 18 digits. Input X is Double Precision X - Argument, X.gt.0.0D0 Output DGAMRN is DOUBLE PRECISION DGAMRN - Ratio GAMMA(X)/GAMMA(X+0.5)

SEE ALSO
DBSKIN

REFERENCES
Y. L. Luke, The Special Functions and Their Approximations, Vol. 1, Math In Sci. And Eng. Series 53, Academic Press, New York, 1969, pp. 34-35.

ROUTINES CALLED
D1MACH, I1MACH

REVISION HISTORY (YYMMDD)
820601 DATE WRITTEN
890531 Changed all specific intrinsics to generic. (WRB)
890911 Removed unnecessary intrinsics. (WRB)
891214 Prologue converted to Version 4.0 format. (BAB)
900328 Added TYPE section. (WRB)
910722 Updated AUTHOR section. (ALS)
920520 Added REFERENCES section. (WRB)