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Psi Function
Math Engineering Class

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

Compute the Psi (or Digamma) function.

Examples:
    Psi(0.1) = -10.4237549404111
    Psi(0.5) = -1.96351002602142
    Psi(0.9) = -0.754926949947051
    Psi(1.0) = -0.577215664901533
    Psi(1.5) = 3.64899739785765E-02
    Psi(2.0) = 0.422784335098467
    Psi(2.5) = 0.703156640645243
    Psi(3.0) = 0.922784335098467
    Psi(4.0) = 1.2561176684318
    Psi(4.5) = 1.38887092635953
See also:
    Gamma Function
    DPSI Function
vX: Function returns Null if vX is Null or cannot be fixed up to a Double precision floating point number.

Psi calculates the double precision Psi (or Digamma) function for double precision argument X. PSI(X) is the logarithmic derivative of the Gamma function of X.

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