[Plot]

> even*2-periodic*extension*of*f(x) = x^2

[Plot]

> 1/3-4*cos(Pi*x)/(Pi^2)+cos(2*Pi*x)/(Pi^2)-4*cos(3*Pi*x)/(9*Pi^2)

[Plot]

> 1/3-4*cos(Pi*x)/(Pi^2)+cos(2*Pi*x)/(Pi^2)-4*cos(3*Pi*x)/(9*Pi^2)+cos(4*Pi*x)/(4*Pi^2)-4*cos(5*Pi*x)/(25*Pi^2)