OpenLibm/slatec/r9lgic.f
Viral B. Shah c977aa998f Add Makefile.extras to build libopenlibm-extras.
Replace amos with slatec
2012-12-31 16:37:05 -05:00

53 lines
1.5 KiB
Fortran

*DECK R9LGIC
FUNCTION R9LGIC (A, X, ALX)
C***BEGIN PROLOGUE R9LGIC
C***SUBSIDIARY
C***PURPOSE Compute the log complementary incomplete Gamma function
C for large X and for A .LE. X.
C***LIBRARY SLATEC (FNLIB)
C***CATEGORY C7E
C***TYPE SINGLE PRECISION (R9LGIC-S, D9LGIC-D)
C***KEYWORDS COMPLEMENTARY INCOMPLETE GAMMA FUNCTION, FNLIB, LARGE X,
C LOGARITHM, SPECIAL FUNCTIONS
C***AUTHOR Fullerton, W., (LANL)
C***DESCRIPTION
C
C Compute the log complementary incomplete gamma function for large X
C and for A .LE. X.
C
C***REFERENCES (NONE)
C***ROUTINES CALLED R1MACH, XERMSG
C***REVISION HISTORY (YYMMDD)
C 770701 DATE WRITTEN
C 890531 Changed all specific intrinsics to generic. (WRB)
C 890531 REVISION DATE from Version 3.2
C 891214 Prologue converted to Version 4.0 format. (BAB)
C 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ)
C 900720 Routine changed from user-callable to subsidiary. (WRB)
C***END PROLOGUE R9LGIC
SAVE EPS
DATA EPS / 0.0 /
C***FIRST EXECUTABLE STATEMENT R9LGIC
IF (EPS.EQ.0.0) EPS = 0.5*R1MACH(3)
C
XPA = X + 1.0 - A
XMA = X - 1.0 - A
C
R = 0.0
P = 1.0
S = P
DO 10 K=1,200
FK = K
T = FK*(A-FK)*(1.0+R)
R = -T/((XMA+2.0*FK)*(XPA+2.0*FK)+T)
P = R*P
S = S + P
IF (ABS(P).LT.EPS*S) GO TO 20
10 CONTINUE
CALL XERMSG ('SLATEC', 'R9LGIC',
+ 'NO CONVERGENCE IN 200 TERMS OF CONTINUED FRACTION', 1, 2)
C
20 R9LGIC = A*ALX - X + LOG(S/XPA)
C
RETURN
END