int n = 40;
mesh Th=square(n,n,[2*x-1,2*y-1]);
fespace Xh(Th,P2);
Xh x2=x^2,y2=y^2,xy=x*y;
Xh w,ww;

w=dxx(x2); cout << "dxx(x2) " << w[].max << " " << w[].min << endl;
w=dyy(y2); cout << "dyy(y2) " << w[].max << " " << w[].min << endl;
w=dyy(xy); cout << "dyy(xy) " << w[].max << " " << w[].min << endl;

w=dxy(xy); cout << "dxy(xy) " << w[].max << " " << w[].min << endl;
w=dyx(xy); cout << "dyx(xy) " << w[].max << " " << w[].min << endl;
w=dyx(x2); cout << "dyx(x2) " << w[].max << " " << w[].min << endl;
//  for regtest 
real regvalue=int2d(Th)(w*w);
