clear all; 
tspan=[0 20]; y0=[1 0]; 
f = @(t,y) [y(2); -y(1)]; 
options = odeset('RelTol',1e-8,'AbsTol',[1e-30 1e-30]);
[t,y] = ode45(f,tspan,y0,options); 
plot(t, y(:,1))

