 Solution

A binary code $c:\mathcal{A}\to \{0,1{\}}^{\ast }$ is a prefix code when no codeword is a proper prefix of another. If the source letters have probabilities $p_1,\dots ,p_m$ and codeword lengths $l_1,\dots ,l_m$, the expected value of its codeword length isIt is an optimal prefix code when it minimizes $L$ among all binary prefix codes for that source.

Suppose $p_i>p_j$ but $l_i>l_j$. Exchanging the codewords assigned to ${\mu }_i$ and ${\mu }_j$ leaves the set of words, and hence prefix-freeness, unchanged. The change in expected length iscontradicting optimality. Thuswhich is the probability monotonicity of optimal prefix-code lengths.

Represent the code by its binary prefix tree and choose a codeword $wb$ of maximal length, where $b\in \{0,1\}$. Its sibling is $w(1-b)$. If that sibling were not a codeword, it could not have a codeword below it either, since such a word would be longer than $wb$. Replacing $wb$ by $w$ would then preserve prefix-freeness and strictly reduce $L$. This is impossible for an optimal code, so $w(1-b)$ is also a maximal-length codeword. The pair differs only in its last digit, proving the deepest sibling property of an optimal prefix code.