Suppose that f:Rn→R. Recall that Hf is the symmetric n×n Hessian matrix of f defined (at each point x ) by (Hf(x))ij​=∂xi​∂xj​∂2f​(x) and that for any symmetric n×n matrix A,QA​(v,w)=∑ij​Aij​vi​wj​. Assuming (⋆), prove that Dw​(Dv​f(x))=QHf​(w,v) Hint: We can express the right-hand side of as a double sum. Can you express the left hand side of (\$) as a double sum as well?