<s>[INST] Two sequences defined by recurrence relations satisfy $x_n/y_n<\sqrt{7}$ for all $n$ Let $(x_n)_{n\geq 1}$ and $(y_n)_{n\geq 1}$ be two sequences such that:
$$x_{n+1}=x_n^2+1 \quad \text{ and } \quad y_{n+1}=x_n y_n$$
with $x_1=2$  and   $y_1=1$ 
Prove that for all $n$
$$\dfrac{x_n}{y_n} < \sqrt{7}.$$
Can I have any help with this situation?
 [/INST] Given that:
$$\frac{x_{n+1}}{y_{n+1}}=\frac{x_n}{y_n}+\frac{1}{x_n y_n}$$
we have:
$$\frac{x_{n+1}}{y_{n+1}}=\frac{1}{x_n y_n}+\frac{1}{x_{n-1} y_{n-1}}+\ldots+\frac{1}{x_1 y_1}+\frac{x_1}{y_1}$$
or just:
$$\frac{x_{n+1}}{y_{n+1}}=\frac{1}{x_n\cdot\ldots\cdot x_1}+\frac{1}{x_{n-1}\cdot\ldots\cdot x_1}+\ldots+\frac{1}{x_1}+2.$$
Hence we need to prove that:
$$\sum_{m=1}^{+\infty}\frac{1}{\prod_{n=1}^{m}x_n}\leq\sqrt{7}-2.\tag{1}$$
The form of the last inequality strongly suggest a continued fraction approach. 
However, we can just notice that $x_1=2,x_2=5$ and for any $n\geq 3$ we have: $$x_n\geq 5^{2^{n-2}},$$
giving that the LHS of $(1)$ is less than
$$\frac{1}{2}+\frac{1}{10}+\frac{1}{10}\sum_{k=1}^{+\infty}\frac{1}{5^{2^k}}\leq \frac{3}{5}+\frac{1}{10}\sum_{k=1}^{+\infty}\frac{1}{5^{k+1}}=\frac{121}{200}.$$
Since $\frac{521}{200}<\sqrt{7}$, we're done.
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