4. The displacement y(t) of an undamped oscillator of mass m>0 on a spring with spring constant k>0, and initial displacement a 0 and initial velocity zero satisfies my" ky 0, y(0) = a, y'(0) = 0 a. Solve this initial value problem. b. Show that the solution is periodic with period T, meaning that y(t+7)= y(t), and express T in terms of m and k.