function M=DF(x,y)
      t2 = x^2;
      t5 = y^2;
      t14 = 3+t2+t5;
      t15 = t14^2;
      t19 = 2*(-27*x+3*t2*x-9*t5*x+3*t2-3*y*t2-3-t5+3*y+t5*y)/t15/t14;
      M(1,1)=t19;
      t2 = x^2;
      t5 = y^2;
      t14 = 3+t2+t5;
      t15 = t14^2;
      t20 = -2*(-9*y-3*y*t2+t5*y-3*t5-9*t5*x+3+t2+9*x+3*t2*x)/t15/t14;
      M(2,2)=t20;
      t2 = x^2;
      t5 = y^2;
      t15 = 3+t2+t5;
      t16 = t15^2;
      t20 = 2*(-9*y+9*y*t2-3*t5*y+3*x+t2*x-3*t5*x+4*x*y)/t16/t15;
      M(2,1)=t20;
      M(1,2)=t20; 
end
