function mm=runge(n)
##n=12; 
i=1:n+1; a=-5; b=5;
f=@(x) 1./(1+x.^2);
## equispaced
x=a+(i-1)*(b-a)/n;
## chebyshev
x=(a+b)/2-(b-a)/2*cos((2*i-1)*pi/(2*n+2));
p=polyfit(x,f(x),n);
m=5000; j=1:m+1;
xp=a+(j-1)*(b-a)/m;
yp=polyval(p,xp);
ys=spline(x,f(x),xp);
plot(xp,ys,"-r","linewidth", 2,x,f(x),"ob","linewidth", 2,xp,f(xp),"-b","linewidth", 2)
axis([-5 5 -5 5])
mm=norm(f(xp)-ys,inf);
end