[_elite]

clc
clear
xk=[0,1,5,8,9,11,13,14,15,16,20];
x=0:0.01:xk(end); y=zeros(size(x));
a=2;
for i=1length(xk)-1)
y=y+(-1)^i*a*(x>=xk(i) &x<xk(i+1));
end
plot(x,y)
axis([0,20 ,-3 3])