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For discrete random variables, write $p(u,v)$ for the joint probability mass function and $p_U(u)$ for its marginal. The conditional entropy is the average of the entropies of the conditional distributions:Conditional distributions at zero-probability values of $u$ can be chosen arbitrarily; they contribute nothing. Here $h$ denotes Shannon entropy, with one common logarithm base and $0\log 0=0$.

For $p(u,v)>0$, use $\log p(u,v)=\log p_U(u)+\log p(v\mid u)$ and sum with weight $-p(u,v)$. Summing out $v$ in the first term givesFor countable alphabets, the entropy contributions are nonnegative, so the equality remains valid with extended values by summing nonnegative terms. Applying this identity successively to the prefix $(X_1,\dots ,X_{i-1})$ and $X_i$ proves the chain rule for information entropy:For $i=1$, the conditioning tuple is empty and the term is $h(X_1)$.

For the subset inequalities, first suppose the marginal entropies are finite, so all joint entropies below are finite by the chain rule and the assumed conditioning reduces entropy property. Put $H=h(X_1,\dots ,X_n)$, and let $H_{-i}$ be the joint entropy with coordinate $i$ omitted. Apply the two-variable chain rule in the order $(X_{[n]\setminus \{i\}},X_i)$ to getThe inequality holds because the prefix conditioning set is contained in the set of all other coordinates. Summing over $i$ and using the chain rule givesThis proves Han's entropy inequality. In terms of the normalized subset entropy, $h_n^{(n)}=H/n$ and $h_{n-1}^{(n)}={\sum }_i{H}_{-i}/[n(n-1)]$, hence $\boxed{h_n^{(n)}\le h_{n-1}^{(n)}}$ for $n\ge 2$.

Apply the same result to each $k$-element coordinate set $S$. It givesWhen this is summed over all $k$-subsets, each fixed $(k-1)$-subset $T$ occurs once for each possible added coordinate, namely $n-k+1$ times. ThereforeSubstitute ${\sum }_{|S|=k}h(X_S)=k\left(\genfrac{}{}{0ex}{}{n}{k}\right)h_k^{(n)}$ and the analogous identity at $k-1$. The binomial coefficient identity $k\left(\genfrac{}{}{0ex}{}{n}{k}\right)=(n-k+1)\left(\genfrac{}{}{0ex}{}{n}{k-1}\right)$ cancels the common positive factor and givesThis is the monotonicity of normalized subset entropies, obtained from the local $k$-coordinate inequality by explicit counting.

If some marginal entropy is infinite, each subset average contains at least one subset including that coordinate. The entropy of such a subset is at least that marginal entropy, by the nonnegative conditional-entropy chain rule, so every $h_k^{(n)}$ is infinite. The inequalities then hold in the extended sense. Otherwise the finite-entropy proof applies. For independent random variables the averages are all $n^{-1}{\sum }_{i}h(X_i)$; repeated copies of one variable instead give $h_k^{(n)}=h(X_1)/k$, illustrating the entropy reduction caused by redundancy.