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35 lines
1,022 B
Fortran
35 lines
1,022 B
Fortran
*DECK DEI
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DOUBLE PRECISION FUNCTION DEI (X)
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C***BEGIN PROLOGUE DEI
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C***PURPOSE Compute the exponential integral Ei(X).
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C***LIBRARY SLATEC (FNLIB)
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C***CATEGORY C5
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C***TYPE DOUBLE PRECISION (EI-S, DEI-D)
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C***KEYWORDS EI FUNCTION, EXPONENTIAL INTEGRAL, FNLIB,
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C SPECIAL FUNCTIONS
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C***AUTHOR Fullerton, W., (LANL)
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C***DESCRIPTION
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C
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C DEI calculates the double precision exponential integral, Ei(X), for
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C positive double precision argument X and the Cauchy principal value
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C for negative X. If principal values are used everywhere, then, for
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C all X,
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C
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C Ei(X) = -E1(-X)
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C or
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C E1(X) = -Ei(-X).
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C
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C***REFERENCES (NONE)
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C***ROUTINES CALLED DE1
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C***REVISION HISTORY (YYMMDD)
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C 770701 DATE WRITTEN
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C 891115 Modified prologue description. (WRB)
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C 891115 REVISION DATE from Version 3.2
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C***END PROLOGUE DEI
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DOUBLE PRECISION X, DE1
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C***FIRST EXECUTABLE STATEMENT DEI
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DEI = -DE1(-X)
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C
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RETURN
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END
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