# Proving a number defined by a sequence is a square number

I found this problem in a math magazine:

Given the sequence $(x_n)_{n \in \mathbb{N}}$ defined by: $$x_0 = 0\\ x_1 = 1\\ x_{n+2}+x_{n+1}+2x_{n}=0$$ Prove that $s_n = 2^{n+1}-7x_{n-1}^2, n > 0$ is a square number.

I tried searching a rule between the numbers of the sequence and the square numbers ($s_n$) formed: $$x_2=-1\\ x_3=-1\\ x_4=3\\ x_5=-1\\ x_6=-5\\ x_7=7\\ x_8=3\\ x_9=-17\\$$ $$s_1 = 2^2\\ s_2 = 1^2\\ s_3 = 3^2\\ s_4 = 5^2\\ s_5 = 1^2\\ s_6 = 11^2\\ s_7 = 9^2\\$$ If I rewrite the rule and square it: $$x_{n-1} = -x_{n-2}-2x_{n-3}, n > 3\\ x_{n-1}^2=x_{n-2}^2 + 4x_{n-3}^2 + 4x_{n-2}x_{n-3}\\$$ apply for the next two, $x_{n-2}$, $x_{n-3}$ the same rule: $$x_{n-2}^2=x_{n-3}^2 + 4x_{n-4}^2 + 4x_{n-3}x_{n-4}\\ x_{n-3}^2=x_{n-4}^2 + 4x_{n-5}^2 + 4x_{n-4}x_{n-5}\\$$ substitute: $$x_{n-1}^2=x_{n-3}^2 + 4x_{n-4}^2 + 4x_{n-3}x_{n-4}+4x_{n-3}^2 + 4x_{n-2}x_{n-3}\\ x_{n-1}^2=5x_{n-3}^2 + 4x_{n-4}^2 + 4x_{n-2}x_{n-3} + 4x_{n-3}x_{n-4} \\ x_{n-1}^2=5(x_{n-4}^2 + 4x_{n-5}^2 + 4x_{n-4}x_{n-5}) + 4x_{n-4}^2 + 4x_{n-2}x_{n-3} + 4x_{n-3}x_{n-4}\\ x_{n-1}^2=9x_{n-4}^2 + 20x_{n-5}^2 + 4x_{n-2}x_{n-3} + 4x_{n-3}x_{n-4} + 20x_{n-4}x_{n-5}\\$$ It seems to lead to a dead end.

A help would be really appreciated.

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Note that further to froggie's solution below, you need to be careful to pick the correct sign of the numbers you are squaring eg $s_2=(-1)^2$ - which makes the pattern easier to spot. –  Mark Bennet Dec 15 '12 at 15:48

If you're familiar with solving linear recurrences, you'll know that you can write a formula for the $x_n$ of the form $$x_n = A\lambda_+^n + B\lambda_-^n,$$ where $A$ and $B$ are constants and the $\lambda_\pm$ are the roots of the polynomial $x^2 + x + 2$, that is, $$\lambda_\pm = \frac{-1 \pm i\sqrt{7}}{2}.$$ Using the initial conditions $x_0 = 0$ and $x_1 = 1$, you can solve for the constants $A$ and $B$:$$A = -B = \frac{-i}{\sqrt{7}}.$$ This then gives the explicit formula for the $x_n$:$$x_n = \frac{-i}{\sqrt{7}}(\lambda_+^n - \lambda_-^n).$$ Now one has $$s_n = 2^{n+1} - 7x_{n-1}^2 = 2^{n+1} + (\lambda_+^{n-1} - \lambda_-^{n-1})^2 = 2^{n+1} + \lambda_+^{2(n-1)} + \lambda_-^{2(n-1)} - 2(\lambda_+\lambda_-)^{n-1}.$$ Using the fact that $\lambda_+\lambda_- = 2$, this becomes $$s_n = \lambda_+^{2(n-1)} + \lambda_-^{2(n-1)} + 2^n.$$ But then we can use $\lambda_+\lambda_- = 2$ again to write $$s_n = \lambda_+^{2(n-1)} + \lambda_-^{2(n-1)} + 2(\lambda_+\lambda_-)^{n-1} = (\lambda_+^{n-1} + \lambda_-^{n-1})^2.$$ Now it only remains to show that $\lambda_+^{n-1} + \lambda_-^{n-1}$ is an integer. Define $\alpha_n = \lambda_+^n + \lambda_-^n$ for each $n$. Then $\alpha_0 = 2$, $\alpha_1 = -1$, and $\alpha_{n+2} + \alpha_{n+1} + 2\alpha_n = 0$ for each $n\geq 2$, so $\alpha_n$ is an integer for all $n$. This completes the proof.