1. The problem statement, all variables and given/known data the higher order equation y'' + y = 0 can be written as a first order system by introducing y' as another unknown besides y: 2. Relevant equations d[y y']t/dt = [y' y'']t = [y' -y]t if this is du/dt = Au what is 2x2 matrix A? 3. The attempt at a solution I think it's [0 1] [-1 0] but then I get complex eigenvalues so I am a bit suspicious... could someone verify or help me out? thanks.