  Efficiency at maximum power output of irreversible heat engines has attracted
a lot of interest in recent years. We discuss the occurance of a particularly
simple and elegant formula for this efficiency in various different models. The
so-called Curzon-Ahlborn efficiency is given by the square-root formula:
$1-\sqrt{T_c/T_h}$, where $T_c$ and $T_h$ are the cold and hot reservoir
temperatures.
