#set terminal postscript eps color
#set output "xsquare.eps"
set xrange [-pi:pi]
t(x,k) = 4.0/(k**2)*(-1)**k*cos(k*x)
series(x,n) = (n>0 ? t(x,n) + series(x,n-1) : pi**2/3)
plot series(x,5)
replot x**2