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annotate libcruft/slatec-fn/dgami.f @ 4720:e759d01692db ss-2-1-53

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[project @ 2004-01-23 04:13:37 by jwe]
author jwe
date Fri, 23 Jan 2004 04:13:37 +0000
parents fe6f9bd9d0e6
children
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fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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1
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2 *DECK DGAMI
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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3 DOUBLE PRECISION FUNCTION DGAMI (A, X)
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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4 C***BEGIN PROLOGUE DGAMI
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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5 C***PURPOSE Evaluate the incomplete Gamma function.
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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6 C***LIBRARY SLATEC (FNLIB)
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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7 C***CATEGORY C7E
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8 C***TYPE DOUBLE PRECISION (GAMI-S, DGAMI-D)
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9 C***KEYWORDS FNLIB, INCOMPLETE GAMMA FUNCTION, SPECIAL FUNCTIONS
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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10 C***AUTHOR Fullerton, W., (LANL)
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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11 C***DESCRIPTION
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12 C
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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13 C Evaluate the incomplete gamma function defined by
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14 C
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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15 C DGAMI = integral from T = 0 to X of EXP(-T) * T**(A-1.0) .
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16 C
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17 C DGAMI is evaluated for positive values of A and non-negative values
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18 C of X. A slight deterioration of 2 or 3 digits accuracy will occur
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19 C when DGAMI is very large or very small, because logarithmic variables
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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20 C are used. The function and both arguments are double precision.
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21 C
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22 C***REFERENCES (NONE)
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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23 C***ROUTINES CALLED DGAMIT, DLNGAM, XERMSG
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24 C***REVISION HISTORY (YYMMDD)
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25 C 770701 DATE WRITTEN
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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26 C 890531 Changed all specific intrinsics to generic. (WRB)
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27 C 890531 REVISION DATE from Version 3.2
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28 C 891214 Prologue converted to Version 4.0 format. (BAB)
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29 C 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ)
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30 C***END PROLOGUE DGAMI
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31 DOUBLE PRECISION A, X, FACTOR, DLNGAM, DGAMIT
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32 C***FIRST EXECUTABLE STATEMENT DGAMI
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33 IF (A .LE. 0.D0) CALL XERMSG ('SLATEC', 'DGAMI',
fe6f9bd9d0e6 [project @ 1997-11-26 07:52:06 by jwe]
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34 + 'A MUST BE GT ZERO', 1, 2)
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35 IF (X .LT. 0.D0) CALL XERMSG ('SLATEC', 'DGAMI',
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36 + 'X MUST BE GE ZERO', 2, 2)
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37 C
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38 DGAMI = 0.D0
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39 IF (X.EQ.0.0D0) RETURN
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40 C
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41 C THE ONLY ERROR POSSIBLE IN THE EXPRESSION BELOW IS A FATAL OVERFLOW.
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42 FACTOR = EXP (DLNGAM(A) + A*LOG(X))
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43 C
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44 DGAMI = FACTOR * DGAMIT (A, X)
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45 C
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46 RETURN
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47 END