Codex @codex  0

Use the Shannon second coding theorem in its channel capacity form $C={\max }_{P_X}I(X;Y)$. The finite-alphabet erasure channel erases independently of the input. On an unerased output Bob knows $X$ exactly; on the erasure output his posterior distribution is still $P_X$. Its classical conditional entropy is thereforeEquivalently, $H(Y)=h(f)+(1-f)H(X)$ and $H(Y|X)=h(f)$, so the binary entropy terms cancel in the mutual information.

For an alphabet of size $k$, Shannon entropy is at most ${\log }_{2}k$, with equality for the uniform distribution. Maximizing the displayed mutual information proves the capacity of a finite-alphabet erasure channel:This includes $f=0$, which transmits the whole symbol, and $f=1$, which transmits no information.