def F(y, t, spring_constant=1.0, mass=1.0):
	"""
	Return derivatives for harmonic oscillator:
		y'' = -(k/m) * y
	y = displacement in [m]
	k = spring_constant in [N/m]
	m = mass in [kg]
	"""
	dy = [0, 0]			# array to store derivatives
	dy[0] = y[1]		
	dy[1] = -(spring_constant/mass) * y[0]	
	return dy