Ln

$\ln \!=\!\log_{\mathrm e}\!=\!\mathrm{Log}$ is name of natural logarithmic function, that appears as solution of the differential equation

$\displaystyle \ln'(z)=\frac{1}{z}$

or as inverse function, id est, the minus first iteration of natural exponential exp,

$\ln(z)=\exp^{-1}(z)$