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EI Function
Math Engineering Class

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

Compute the exponential integral Ei(X).

Examples: 
    EI(-5.0) = -1.14829559127533E-03
    EI(-5.0) = -E1(+5.0)
    EI(-0.5) = -0.559773594776161
    IsNull(EI(0)) = True
    EI(+0.1) = -1.62281281396928
    EI(+0.5) = 0.454219904863174
    EI(+1.0) = 1.89511781635594
    EI(+5.0) = 40.1852753558032
    EI(+5.0) = -E1(-5.0)
See also: 
    E1 Function
    DEI Function
vX: Function returns Null if vX is Null or cannot be fixed up to a Double precision floating point number.

EI calculates the double precision exponential integral, Ei(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, 

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

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