function z=fct2D(x,y)  % the standard given function,                        % also used for solving the Poisson equation
global func3d
switch func3d
case 1
 z=abs(6-x.^2-y.^2) ;%1
case 2
z=peaks(x,y);
case 3%2
 z=x.^2+y.^2; 
case 4%3    z=exp(-x.^2-y.^2);case 5
 z=exp(-x.^2-5*y.^2);
end