function sol = exer6
% exer6, exer6f, exer6h of tutorial.

% Sample problem of ARCHI manual.  The absolute error 
% tolerance is set to 1e-9 to illustrate the use of 
% options.  Also, it is necessary to use the InitialY 
% option because the solution is not continuous at the
% initial point.

options = ddeset('AbsTol',1e-9,'InitialY',[0; 0]);
sol = dde23(@exer6f,[1, 2],@exer6h,[0, 4],options);
plot(sol.x,sol.y)
title('Sample Problem of ARCHI Manual.')
xlabel('time t');
ylabel('y(t)');

%=================================================
function yp = exer6f(t,y,Z)
ylag1 = Z(:,1);
ylag2 = Z(:,2);
yp = [  ylag1(1)*ylag2(2)
       -y(1)*ylag2(2)     ];

%=================================================
function y = exer6h(t)
y = [cos(t); sin(t)];