I'm suck with the following question
I'm asked to find all possible real values of $a$ for which the solution to the following ODE is periodic. The ODE is: x''=-\nabla U(x),\quad x\in\mathbb R^2. We have then two cases:
a) $U(x)=x_1^2+ax_2^2$ and
b) $U(x)=x_1^2+x_2^2+ax_1x_2.$
For case a) the equation is equivalent to x_1''-2x_1=0 which leads to $x_1=C_1\cos(\sqrt2t)+C_2\sin(\sqrt2 t)$ and x_2''-2ax_2=0, which forces $a$ to be grater than $0$ for the component $x_2$ to be periodic. All this leads to $x_2=C_3\cos(\sqrt{2a}t)+C_4\sin(\sqrt{2a}t).$ For $x=(x_1,x_2)$ to be periodic, it should be true, even if i'm not sure, that the ratio of the two periods must be rational. If this were true then i should have $\sqrt a\in\mathbb Q$, but this forces $a\in\mathbb Q^2$. Am i right?
For case b) i can write the equation as \left(\begin{array}{c}x_1''\\ x_2''\end{array}\right)=\left(\begin{array}{cc}2 & a\\ a & 2\end{array}\right)\cdot\left(\begin{array}{c}x_1\\ x_2\end{array}\right). Where to go from here, i've never found myself in a similar situation. Do i have to diagonalize the matrix? And then? Really, I am new to these things. Any help is greatly appreciated.