mirror of
https://git.planet-casio.com/Lephenixnoir/OpenLibm.git
synced 2025-01-01 06:23:39 +01:00
98 lines
2.8 KiB
Fortran
98 lines
2.8 KiB
Fortran
*DECK D9GMIC
|
|
DOUBLE PRECISION FUNCTION D9GMIC (A, X, ALX)
|
|
C***BEGIN PROLOGUE D9GMIC
|
|
C***SUBSIDIARY
|
|
C***PURPOSE Compute the complementary incomplete Gamma function for A
|
|
C near a negative integer and X small.
|
|
C***LIBRARY SLATEC (FNLIB)
|
|
C***CATEGORY C7E
|
|
C***TYPE DOUBLE PRECISION (R9GMIC-S, D9GMIC-D)
|
|
C***KEYWORDS COMPLEMENTARY INCOMPLETE GAMMA FUNCTION, FNLIB, SMALL X,
|
|
C SPECIAL FUNCTIONS
|
|
C***AUTHOR Fullerton, W., (LANL)
|
|
C***DESCRIPTION
|
|
C
|
|
C Compute the complementary incomplete gamma function for A near
|
|
C a negative integer and for small X.
|
|
C
|
|
C***REFERENCES (NONE)
|
|
C***ROUTINES CALLED D1MACH, DLNGAM, XERMSG
|
|
C***REVISION HISTORY (YYMMDD)
|
|
C 770701 DATE WRITTEN
|
|
C 890531 Changed all specific intrinsics to generic. (WRB)
|
|
C 890911 Removed unnecessary intrinsics. (WRB)
|
|
C 890911 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 D9GMIC
|
|
DOUBLE PRECISION A, X, ALX, ALNG, BOT, EPS, EULER, FK, FKP1, FM,
|
|
1 S, SGNG, T, TE, D1MACH, DLNGAM
|
|
LOGICAL FIRST
|
|
SAVE EULER, EPS, BOT, FIRST
|
|
DATA EULER / 0.5772156649 0153286060 6512090082 40 D0 /
|
|
DATA FIRST /.TRUE./
|
|
C***FIRST EXECUTABLE STATEMENT D9GMIC
|
|
IF (FIRST) THEN
|
|
EPS = 0.5D0*D1MACH(3)
|
|
BOT = LOG (D1MACH(1))
|
|
ENDIF
|
|
FIRST = .FALSE.
|
|
C
|
|
IF (A .GT. 0.D0) CALL XERMSG ('SLATEC', 'D9GMIC',
|
|
+ 'A MUST BE NEAR A NEGATIVE INTEGER', 2, 2)
|
|
IF (X .LE. 0.D0) CALL XERMSG ('SLATEC', 'D9GMIC',
|
|
+ 'X MUST BE GT ZERO', 3, 2)
|
|
C
|
|
M = -(A - 0.5D0)
|
|
FM = M
|
|
C
|
|
TE = 1.0D0
|
|
T = 1.0D0
|
|
S = T
|
|
DO 20 K=1,200
|
|
FKP1 = K + 1
|
|
TE = -X*TE/(FM+FKP1)
|
|
T = TE/FKP1
|
|
S = S + T
|
|
IF (ABS(T).LT.EPS*S) GO TO 30
|
|
20 CONTINUE
|
|
CALL XERMSG ('SLATEC', 'D9GMIC',
|
|
+ 'NO CONVERGENCE IN 200 TERMS OF CONTINUED FRACTION', 4, 2)
|
|
C
|
|
30 D9GMIC = -ALX - EULER + X*S/(FM+1.0D0)
|
|
IF (M.EQ.0) RETURN
|
|
C
|
|
IF (M.EQ.1) D9GMIC = -D9GMIC - 1.D0 + 1.D0/X
|
|
IF (M.EQ.1) RETURN
|
|
C
|
|
TE = FM
|
|
T = 1.D0
|
|
S = T
|
|
MM1 = M - 1
|
|
DO 40 K=1,MM1
|
|
FK = K
|
|
TE = -X*TE/FK
|
|
T = TE/(FM-FK)
|
|
S = S + T
|
|
IF (ABS(T).LT.EPS*ABS(S)) GO TO 50
|
|
40 CONTINUE
|
|
C
|
|
50 DO 60 K=1,M
|
|
D9GMIC = D9GMIC + 1.0D0/K
|
|
60 CONTINUE
|
|
C
|
|
SGNG = 1.0D0
|
|
IF (MOD(M,2).EQ.1) SGNG = -1.0D0
|
|
ALNG = LOG(D9GMIC) - DLNGAM(FM+1.D0)
|
|
C
|
|
D9GMIC = 0.D0
|
|
IF (ALNG.GT.BOT) D9GMIC = SGNG * EXP(ALNG)
|
|
IF (S.NE.0.D0) D9GMIC = D9GMIC +
|
|
1 SIGN (EXP(-FM*ALX+LOG(ABS(S)/FM)), S)
|
|
C
|
|
IF (D9GMIC .EQ. 0.D0 .AND. S .EQ. 0.D0) CALL XERMSG ('SLATEC',
|
|
+ 'D9GMIC', 'RESULT UNDERFLOWS', 1, 1)
|
|
RETURN
|
|
C
|
|
END
|