Im kind of lost on linearizing the ODE $\displaystyle y''+\arctan(y)=0$

I should convert this to something like y'=Ay

I guess withy'=(y',y'') andy=(y,y'). Can someone refresh my memory a little? ;p

[edit] I found out I have to convert the system into something like this

$\displaystyle x_1=y, x_2=y', x_1'=x_2, x_2'=-\arctan(x_1)$