# dei.f

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