set terminal postscript eps color
set output "xsquare.eps"
t(x,k) = 4.0/(k**2)*(-1)**k*cos(k*x)
series(x,n) = (n>1 ? t(x,n) + series(x,n-1) : t(x,1))
plot [-pi:pi] x**2, pi**2/3 + series(x,5)