
function dx = simple_observer(t,x)
a1=0.1;a2=0;;k1=0.5;k2=0.5;
dx = [0 0 0 0]';    % a column vector
dx(1) = x(2);
dx(2) = -a1*x(1)- a2*x(2)+sin(pi*t);
dx(3) = x(4)+k1*(x(2)-x(4));
dx(4) = -a1*x(3)- a2*x(4)+1*sin(pi*t)+k2*(x(2)-x(4));

%[t,W]=ode45(@rigid,[0 45],[3 -4 0 0]);
%plot(t,W(:,1),t,W(:,2),t,W(:,3),t,W(:,4))
%semilogy(t,abs(W(:,1)-W(:,3)),t,abs(W(:,2)-W(:,4)))
