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Let $$h(p) = -p\log_2(p) - (1-p)\log_2(1-p)$$ be the binary entropy function and $$H(p,q) = -p\log_2(p) -q\log_2(q) - (1-p-q)\log_2(1-p-q)$$ be the "ternary" entropy function. I would like to prove that for all possible values of $p \in [0,1/2]$ and $q \in [0,1/2]$, $$H(p,q) \leq h\bigg(\frac{1-p}{2}\bigg)+h\bigg(\frac{1-q}{2}\bigg).$$ Any help will be appreciated !

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Let's write entropy in the standard form $H(p_1,..,p_n) = - \sum_i p_i\log_2(p_i)$ with $\sum_i p_i = 1$. We can make use of the sub-additivity of entropy (see e.g. here) to write

$$ H(a,1-a) + H(b,1-b) \geq H(0,a,b,1-a-b) $$ Further, $H(0,a,b,1-a-b) = H(a,b,1-a-b)$. Now set $a = (1-p)/2$ and $b = (1-q)/2$. So it suffices to show

$$ D(p,q) = H((1-p)/2, (1-q)/2, 1-(1-p)/2 - (1-q)/2)- H(p,q,1-p-q) \geq 0 $$

Now $D$ has one unique minimum since the two equations $d D / dp = 0$ and $d D / dq = 0$ have only one solution at $p=q=1/3$ with $D(1/3,1/3) = 0$, and since $D>0$ on the boundaries of the required area $[p=0, p=1/2, q = 0, q = 1/2]$.

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