Iterated Entropy Derivatives and Binary Entropy Inequalities

Abstract: We embark on a systematic study of the $(k+1)$-th derivative of $x<sup>{k-r}H(x<sup>r)$, where $H(x):=-x\log x-(1-x)\log(1-x)$ is the binary entropy and $k&gt;r\geq 1$ are integers. Our motivation is the conjectural entropy inequality $\alpha_k H(x^k)\geq x<sup>{k-1}H(x)$, where $0&lt;\alpha_k&lt;1$ is given by a functional equation. The $k=2$ case was the key technical tool driving recent breakthroughs on the union-closed sets conjecture. We express $$\frac{d<sup>{k+1}}{dx<sup>{k+1}}x<sup>{k-r}H(x<sup>r)$$ as a rational function, an infinite series, and a sum over generalized Stirling numbers. This allows us to reduce the proof of the entropy inequality for real $k$ to showing that an associated polynomial has only two real roots in the interval $(0,1)$, which also allows us to prove the inequality for fractional exponents such as $k=3/2$. The proof suggests a new framework for proving tight inequalities for the sum of polynomials times the logarithms of polynomials, which converts the inequality into a statement about the real roots of a simpler associated polynomial.