# Calculate the gradient of the function f x1 x2 x3 2x51x32x43 x1x2 x2x3 at 1 1 1 homework help

1. Calculate the gradient of the function f(x1,x2,x3) = 2x51x32x43 +x1x2 +x2x3 at (1,1,1).

2. Let f(x1,x2) = (x1 +x2)4 âˆ’8(x1 +x2)2. Find all the local minimas and local maximas

of this function. Guess what the graph of this function looks like.

3. Let x âˆˆ R2. Consider the function f(x) = xTAx. If A is an invertible matrix, then prove that this function has only one stationary point at 0(stationary points are the points at which the gradient is zero). Give an A for which 0 is the minimizer of f(x). Give an A for which 0 is neither a maximizer nor a minimizer of f(x).

4. Complete the proof of Lemma 7 in Lecture note 2 (posted on our course website).

5. Let x âˆˆ Rp and find the gradient of the following functions:

(a) f1(x) = (xT Ax)2, where A is an n Ã— n matrix. (b) f2(x) = (xT Ax)n, where A is an n Ã— n matrix.

6. Let A âˆˆ RnÃ—p denote a fat matrix, i.e., n < p. Explain why we should expect the equation y = Ax to have infinitely many solutions. Among all those solutions we would like to find the one with minimum Euclidean norm, i.e., we want to find the solution with the smallest xT x. Find that solution and prove your answer.

7. Plotthefunctionf(x1,x2)=(x1âˆ’1)2+(x2âˆ’1)2+x1x2 inR.Assumethatx1 âˆˆ[âˆ’2,2] and x2 âˆˆ [âˆ’2, 2]. Before doing this problem you may want to study the commands â€œouterâ€ and â€œperspâ€ in R.