the higher order equation y'' + y = 0 can be written as a first order system by introducing y' as another unknown besides y:
d[y y']t/dt = [y' y'']t = [y' -y]t
if this is du/dt = Au what is 2x2 matrix A?
The Attempt at a Solution
I think it's
but then I get complex eigenvalues so I am a bit suspicious... could someone verify or help me out?