1*DECK R9LGIC
2      FUNCTION R9LGIC (A, X, ALX)
3C***BEGIN PROLOGUE  R9LGIC
4C***SUBSIDIARY
5C***PURPOSE  Compute the log complementary incomplete Gamma function
6C            for large X and for A .LE. X.
7C***LIBRARY   SLATEC (FNLIB)
8C***CATEGORY  C7E
9C***TYPE      SINGLE PRECISION (R9LGIC-S, D9LGIC-D)
10C***KEYWORDS  COMPLEMENTARY INCOMPLETE GAMMA FUNCTION, FNLIB, LARGE X,
11C             LOGARITHM, SPECIAL FUNCTIONS
12C***AUTHOR  Fullerton, W., (LANL)
13C***DESCRIPTION
14C
15C Compute the log complementary incomplete gamma function for large X
16C and for A .LE. X.
17C
18C***REFERENCES  (NONE)
19C***ROUTINES CALLED  R1MACH, XERMSG
20C***REVISION HISTORY  (YYMMDD)
21C   770701  DATE WRITTEN
22C   890531  Changed all specific intrinsics to generic.  (WRB)
23C   890531  REVISION DATE from Version 3.2
24C   891214  Prologue converted to Version 4.0 format.  (BAB)
25C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
26C   900720  Routine changed from user-callable to subsidiary.  (WRB)
27C***END PROLOGUE  R9LGIC
28      SAVE EPS
29      DATA EPS / 0.0 /
30C***FIRST EXECUTABLE STATEMENT  R9LGIC
31      IF (EPS.EQ.0.0) EPS = 0.5*R1MACH(3)
32C
33      XPA = X + 1.0 - A
34      XMA = X - 1.0 - A
35C
36      R = 0.0
37      P = 1.0
38      S = P
39      DO 10 K=1,200
40        FK = K
41        T = FK*(A-FK)*(1.0+R)
42        R = -T/((XMA+2.0*FK)*(XPA+2.0*FK)+T)
43        P = R*P
44        S = S + P
45        IF (ABS(P).LT.EPS*S) GO TO 20
46 10   CONTINUE
47      CALL XERMSG ('SLATEC', 'R9LGIC',
48     +   'NO CONVERGENCE IN 200 TERMS OF CONTINUED FRACTION', 1, 2)
49C
50 20   R9LGIC = A*ALX - X + LOG(S/XPA)
51C
52      RETURN
53      END
54