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E1 Function
Math Engineering Class

Public Function E1( _
    ByVal vX As Variant _
    ) As Variant

Compute the exponential integral E1(X).

Examples: 
    E1(-5.0) = -40.1852753558032
    E1(-0.5) = -0.454219904863174
    IsNull(E1(0)) = True
    E1(+0.1) = 1.82292395841939
    E1(+0.5) = 0.559773594776161
    E1(+1.0) = 0.21938393439552
    E1(+5.0) = 1.14829559127533E-03
See also: 
    EI Function
    DE1 Function
vX: Function returns Null if vX is Null or cannot be fixed up to a Double precision floating point number.

E1 calculates the double precision exponential integral, E1(X), for positive double precision argument X and the Cauchy principal value for negative X. If principal values are used everywhere, then, for all X, 

    E1(X) = -Ei(-X)
    or
    Ei(X) = -E1(-X).

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