If $u\in L^1(0,1)$ is nonnegative and $E_n = \int_0^1 x^n u(x) \, dx$, prove $E_{n-k} E_k \leq E_0 E_n$.

Holder's Inequality: Assume, wlog, that $k\geqslant n-k$. $$ \begin{align} \Big(\int_0^1 x^{n-k}u~\text dx\Big)\Big(\int_0^1x^k u~\text dx\Big) &= \Big(\int_0^1 (x^k)^{\frac{n-k}{k}}u~\text dx\Big)\Big(\int_0^1x^k u~\text dx\Big)\\ &\leqslant \Big(\int_0^1 u~\text dx\Big)^{2-\frac{n}{k}}\Big(\int_0^1 x^ku~\text dx\Big)^{\frac{n-k}{k}}\Big(\int_0^1x^k u~\text dx\Big) \\ & = \Big(\int_0^1 u~\text dx\Big)^{2-\frac{n}{k}}\Big(\int_0^1 (x^n)^{\frac{k}{n}}u~\text dx\Big)^{\frac{n}{k}} \\ & \leqslant \Big(\int_0^1 u~\text dx\Big)^{2-\frac{n}{k}}\Big(\int_0^1 x^nu~\text dx\Big)\Big(\int_0^1 u~\text dx\Big)^{(1-\frac{k}{n})\frac{n}{k}} \\ & = \Big(\int_0^1 u~\text dx\Big)\Big(\int_0^1 x^nu~\text dx\Big)\,. \end{align} $$

Therefore,

$$\Big(\int_0^1 x^{n-k}u~\text dx\Big)\Big(\int_0^1x^k u~\text dx\Big)\leqslant \Big(\int_0^1 u~\text dx\Big)\Big(\int_0^1 x^nu~\text dx\Big)$$