Finite-alphabet erasure channel
= Finite-alphabet erasure channel
{title2=$Y=X\text{ with probability }1-f,\quad Y=e\text{ with probability }f$}
A finite-alphabet erasure channel transmits the input symbol unchanged with probability $1-f$ and replaces it by a distinct erasure flag with probability $f$, independently of the symbol. Given the flag, the posterior remains the input distribution; otherwise the input is known. Thus its <mutual information> is $(1-f)H(X)$.