  We study the extremality of the BEC and the BSC for Gallager's reliability
function $E_0$ evaluated under the uniform input distribution for binary input
DMCs from the aspect of channel polarization. In particular, we show that
amongst all B-DMCs of a given $E_0(\rho)$ value, for a fixed $\rho \geq 0$, the
BEC and BSC are extremal in the evolution of $E_0$ under the one-step
polarization transformations.
