[Algebra]
Problem: Let $x_{1}, \ldots, x_{100}$ be nonnegative real numbers such that $x_{i}+x_{i+1}+x_{i+2} \leq 1$ for all $i=1, \ldots, 100$ (we put $x_{101}=x_{1}, x_{102}=x_{2}$ ). Find the maximal possible value of the sum

$$
S=\sum_{i=1}^{100} x_{i} x_{i+2}
$$

Solution: Let $x_{2 i}=0, x_{2 i-1}=\frac{1}{2}$ for all $i=1, \ldots, 50$. Then we have $S=50 \cdot\left(\frac{1}{2}\right)^{2}=\frac{25}{2}$. So, we are left to show that $S \leq \frac{25}{2}$ for all values of $x_{i}$ 's satisfying the problem conditions.

Consider any $1 \leq i \leq 50$. By the problem condition, we get $x_{2 i-1} \leq 1-x_{2 i}-x_{2 i+1}$ and $x_{2 i+2} \leq 1-x_{2 i}-x_{2 i+1}$. Hence by the AM-GM inequality we get

$$
\begin{aligned}
x_{2 i-1} x_{2 i+1} & +x_{2 i} x_{2 i+2} \leq\left(1-x_{2 i}-x_{2 i+1}\right) x_{2 i+1}+x_{2 i}\left(1-x_{2 i}-x_{2 i+1}\right) \\
& =\left(x_{2 i}+x_{2 i+1}\right)\left(1-x_{2 i}-x_{2 i+1}\right) \leq\left(\frac{\left(x_{2 i}+x_{2 i+1}\right)+\left(1-x_{2 i}-x_{2 i+1}\right)}{2}\right)^{2}=\frac{1}{4} .
\end{aligned}
$$

Summing up these inequalities for $i=1,2, \ldots, 50$, we get the desired inequality

$$
\sum_{i=1}^{50}\left(x_{2 i-1} x_{2 i+1}+x_{2 i} x_{2 i+2}\right) \leq 50 \cdot \frac{1}{4}=\frac{25}{2}
$$
We present another proof of the estimate. From the problem condition, we get

$$
\begin{aligned}
S=\sum_{i=1}^{100} x_{i} x_{i+2} \leq \sum_{i=1}^{100} x_{i}\left(1-x_{i}-x_{i+1}\right) & =\sum_{i=1}^{100} x_{i}-\sum_{i=1}^{100} x_{i}^{2}-\sum_{i=1}^{100} x_{i} x_{i+1} \\
& =\sum_{i=1}^{100} x_{i}-\frac{1}{2} \sum_{i=1}^{100}\left(x_{i}+x_{i+1}\right)^{2}
\end{aligned}
$$

By the AM-QM inequality, we have $\sum\left(x_{i}+x_{i+1}\right)^{2} \geq \frac{1}{100}\left(\sum\left(x_{i}+x_{i+1}\right)\right)^{2}$, so

$$
\begin{aligned}
S \leq \sum_{i=1}^{100} x_{i}-\frac{1}{200}\left(\sum_{i=1}^{100}\left(x_{i}+x_{i+1}\right)\right)^{2} & =\sum_{i=1}^{100} x_{i}-\frac{2}{100}\left(\sum_{i=1}^{100} x_{i}\right)^{2} \\
& =\frac{2}{100}\left(\sum_{i=1}^{100} x_{i}\right)\left(\frac{100}{2}-\sum_{i=1}^{100} x_{i}\right) .
\end{aligned}
$$

And finally, by the AM-GM inequality

$$
S \leq \frac{2}{100} \cdot\left(\frac{1}{2}\left(\sum_{i=1}^{100} x_{i}+\frac{100}{2}-\sum_{i=1}^{100} x_{i}\right)\right)^{2}=\frac{2}{100} \cdot\left(\frac{100}{4}\right)^{2}=\frac{25}{2}
$$

Final answer: $\frac{25}{2}$