function Ln (x: Real): Real;
or
function Ln (z: Complex): Complex;
The natural logarith Ln is the logarithm with base e, where e is the Euler number e = Exp (1) = 2.718281828...
The function Ln is defined in ISO 7185 Pascal; its application to complex values is defined in ISO 10206 Extended Pascal.
program LnDemo; var z: Complex; begin z := Cmplx (1, 1); z := Ln (z) { yields Ln (SqRt (2)) + i * Pi / 4 } { since Ln (i * x) = Ln Abs (x) + i * Arg (x) } end.