Identric mean

The identric mean of two positive real numbers x, y is defined as:

\begin{align} I(x,y) &= \frac{1}{e}\cdot \lim_{(\xi,\eta)\to(x,y)} \sqrt[\xi-\eta]{\frac{\xi^\xi}{\eta^\eta}} \\[8pt] &= \lim_{(\xi,\eta)\to(x,y)} \exp\left(\frac{\xi\cdot\ln\xi-\eta\cdot\ln\eta}{\xi-\eta}-1\right) \\[8pt] &= \begin{cases} x & \text{if }x=y \\[8pt] \frac{1}{e} \sqrt[x-y]{\frac{x^x}{y^y}} & \text{else} \end{cases} \end{align} $$

It can be derived from the mean value theorem by considering the secant of the graph of the function $$x \mapsto x\cdot \ln x$$. It can be generalized to more variables according by the mean value theorem for divided differences. The identric mean is a special case of the Stolarsky mean.