00001 *DECK GAMI 00002 FUNCTION GAMI (A, X) 00003 C***BEGIN PROLOGUE GAMI 00004 C***PURPOSE Evaluate the incomplete Gamma function. 00005 C***LIBRARY SLATEC (FNLIB) 00006 C***CATEGORY C7E 00007 C***TYPE SINGLE PRECISION (GAMI-S, DGAMI-D) 00008 C***KEYWORDS FNLIB, INCOMPLETE GAMMA FUNCTION, SPECIAL FUNCTIONS 00009 C***AUTHOR Fullerton, W., (LANL) 00010 C***DESCRIPTION 00011 C 00012 C Evaluate the incomplete gamma function defined by 00013 C 00014 C GAMI = integral from T = 0 to X of EXP(-T) * T**(A-1.0) . 00015 C 00016 C GAMI is evaluated for positive values of A and non-negative values 00017 C of X. A slight deterioration of 2 or 3 digits accuracy will occur 00018 C when GAMI is very large or very small, because logarithmic variables 00019 C are used. GAMI, A, and X are single precision. 00020 C 00021 C***REFERENCES (NONE) 00022 C***ROUTINES CALLED ALNGAM, GAMIT, XERMSG 00023 C***REVISION HISTORY (YYMMDD) 00024 C 770701 DATE WRITTEN 00025 C 890531 Changed all specific intrinsics to generic. (WRB) 00026 C 890531 REVISION DATE from Version 3.2 00027 C 891214 Prologue converted to Version 4.0 format. (BAB) 00028 C 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ) 00029 C***END PROLOGUE GAMI 00030 C***FIRST EXECUTABLE STATEMENT GAMI 00031 IF (A .LE. 0.0) CALL XERMSG ('SLATEC', 'GAMI', 00032 + 'A MUST BE GT ZERO', 1, 2) 00033 IF (X .LT. 0.0) CALL XERMSG ('SLATEC', 'GAMI', 00034 + 'X MUST BE GE ZERO', 2, 2) 00035 C 00036 GAMI = 0.0 00037 IF (X.EQ.0.0) RETURN 00038 C 00039 C THE ONLY ERROR POSSIBLE IN THE EXPRESSION BELOW IS A FATAL OVERFLOW. 00040 FACTOR = EXP (ALNGAM(A) + A*LOG(X) ) 00041 C 00042 GAMI = FACTOR * GAMIT(A, X) 00043 C 00044 RETURN 00045 END
1.7.1
