What is the Hessian matrix HfH_fHf of f(x,y)=x3+y3f(x, y) = x^3 + y^3f(x,y)=x3+y3 at the point (1,1)(1, 1)(1,1)?
(6006)\begin{pmatrix} 6 & 0 \\ 0 & 6 \end{pmatrix}(6006)
(3003)\begin{pmatrix} 3 & 0 \\ 0 & 3 \end{pmatrix}(3003)
(1001)\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}(1001)
(6116)\begin{pmatrix} 6 & 1 \\ 1 & 6 \end{pmatrix}(6116)