> N[11]:=(xi,eta) -> 9*(1-xi)*(9*xi^2-1)*(1+3*eta)*(1-eta^2)/256;

  N[11] := (xi, eta) ->

                            2                          2
        9/256 (1 - xi) (9 xi  - 1) (1 + 3 eta) (1 - eta )

> plot3d(N[11],-1..1,-1..1,grid=[20,20],axes=boxed,scaling=constrained,
> style=patch);

> 
> 
