D = -2*pi:pi/20:2*pi;
plot(D, cos(D), 'b');
hold on
plot(-pi/4:2*pi:2*pi+pi/4, cos(-pi/4:2*pi:2*pi+pi/4), '+r');
plot(-2*pi+pi/4:2*pi:2*pi-pi/4, cos(-2*pi+pi/4:2*pi:2*pi-pi/4), '+r');