% Plot the trajectory of Anti-Gradient Descent
[x,y]=meshgrid(-20:.3:21);
z=-20*exp(-0.2*(0.5*(x.^2+y.^2)).^(1/2))-exp(0.5*(cos(2*pi*x)+cos(2*pi*y)));
figure(1)
hold off
contour(x,y,z)
[dx,dy]=gradient(z,.2,.2);
hold on
quiver(x,y,dx,dy)
% set the initial point
x=[1,0];
xs=x; % dummy variable required for the iterative process
itsk=50; % the number of iterations
step=0.1; % the step size
for i=1:itsk
% compute the next point
x = xs - step*gradient1(xs);
% plot the step
plot([xs(1),x(1)],[xs(2),x(2)],'r',[xs(1),x(1)],[xs(2),x(2)],'ro')
% refresh the variables
xs=x;
end