relibc_openlibm/slatec/d9gmit.f

*DECK D9GMIT
      DOUBLE PRECISION FUNCTION D9GMIT (A, X, ALGAP1, SGNGAM, ALX)
C***BEGIN PROLOGUE  D9GMIT
C***SUBSIDIARY
C***PURPOSE  Compute Tricomi's incomplete Gamma function for small
C            arguments.
C***LIBRARY   SLATEC (FNLIB)
C***CATEGORY  C7E
C***TYPE      DOUBLE PRECISION (R9GMIT-S, D9GMIT-D)
C***KEYWORDS  COMPLEMENTARY INCOMPLETE GAMMA FUNCTION, FNLIB, SMALL X,
C             SPECIAL FUNCTIONS, TRICOMI
C***AUTHOR  Fullerton, W., (LANL)
C***DESCRIPTION
C
C Compute Tricomi's incomplete gamma function 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  D9GMIT
      DOUBLE PRECISION A, X, ALGAP1, SGNGAM, ALX, AE, AEPS, ALGS, ALG2,
     1  BOT, EPS, FK, S, SGNG2, T, TE, D1MACH, DLNGAM
      LOGICAL FIRST
      SAVE EPS, BOT, FIRST
      DATA FIRST /.TRUE./
C***FIRST EXECUTABLE STATEMENT  D9GMIT
      IF (FIRST) THEN
         EPS = 0.5D0*D1MACH(3)
         BOT = LOG (D1MACH(1))
      ENDIF
      FIRST = .FALSE.
C
      IF (X .LE. 0.D0) CALL XERMSG ('SLATEC', 'D9GMIT',
     +   'X SHOULD BE GT 0', 1, 2)
C
      MA = A + 0.5D0
      IF (A.LT.0.D0) MA = A - 0.5D0
      AEPS = A - MA
C
      AE = A
      IF (A.LT.(-0.5D0)) AE = AEPS
C
      T = 1.D0
      TE = AE
      S = T
      DO 20 K=1,200
        FK = K
        TE = -X*TE/FK
        T = TE/(AE+FK)
        S = S + T
        IF (ABS(T).LT.EPS*ABS(S)) GO TO 30
 20   CONTINUE
      CALL XERMSG ('SLATEC', 'D9GMIT',
     +   'NO CONVERGENCE IN 200 TERMS OF TAYLOR-S SERIES', 2, 2)
C
 30   IF (A.GE.(-0.5D0)) ALGS = -ALGAP1 + LOG(S)
      IF (A.GE.(-0.5D0)) GO TO 60
C
      ALGS = -DLNGAM(1.D0+AEPS) + LOG(S)
      S = 1.0D0
      M = -MA - 1
      IF (M.EQ.0) GO TO 50
      T = 1.0D0
      DO 40 K=1,M
        T = X*T/(AEPS-(M+1-K))
        S = S + T
        IF (ABS(T).LT.EPS*ABS(S)) GO TO 50
 40   CONTINUE
C
 50   D9GMIT = 0.0D0
      ALGS = -MA*LOG(X) + ALGS
      IF (S.EQ.0.D0 .OR. AEPS.EQ.0.D0) GO TO 60
C
      SGNG2 = SGNGAM * SIGN (1.0D0, S)
      ALG2 = -X - ALGAP1 + LOG(ABS(S))
C
      IF (ALG2.GT.BOT) D9GMIT = SGNG2 * EXP(ALG2)
      IF (ALGS.GT.BOT) D9GMIT = D9GMIT + EXP(ALGS)
      RETURN
C
 60   D9GMIT = EXP (ALGS)
      RETURN
C
      END