> N[14]:=(xi,eta) -> 81*(1+3*xi)*(1-xi^2)*(1-3*eta)*(1-eta^2)/256;

  N[14] :=

                     81                    2                      2
        (xi, eta) -> --- (1 + 3 xi) (1 - xi ) (1 - 3 eta) (1 - eta )
                     256

> plot3d(N[14],-1..1,-1..1,grid=[20,20],axes=boxed,scaling=constrained,
> style=patch);

> 
> 
