> Phi(xi,eta):=(1-xi^2-eta^2)/2;

                                           2      2
                                         xi    eta
                   Phi(xi, eta) := 1/2 - --- - ----
                                          2     2

> plot3d(Phi(xi,eta),xi=-1..1,eta=-sqrt(1-xi^2)..sqrt(1-xi^2),          
>              axes=boxed);

> 
> 
