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Math W54
• This is Midterm 2. The exam is out of 50 points and has 5 questions, where each question is worth 10 points. You are required to solve all questions. Some of these questions have multiple parts; next to each part, you will find the amount of points the question is worth. • You are required to show your work on each problem on this exam. Mysterious or unsupported answers will not receive full credit. A correct answer, unsupported by explanation or algebraic work will receive no credit; an incorrect answer supported by substantially correct calculations and explanations will receive partial credit. • You must take this exam completely alone. Showing it to or discussing it with anyone else is forbidden. Reproducing or sharing this material on any other websites or platforms is also forbidden. You are permitted to consult your notes, the textbook, any materials handed out in class, and any materials on the bCourses site. You may not consult any other resources (for instance, public websites, Wolfram Alpha, and calculators). Math W54 Name: Practice Midterm 2 July 26, 2022 1. (10 points) Do you think Mind your eigenbusiness would make a cool t-shirt? Solution. Yes. There is no other right answer. Page 2 Math W54 Name: Practice Midterm 2 July 26, 2022 2. In this question, we develop some bounds for the rank of a product! Let A and B be n× n matrices for simplicity. (a) (2 points) Prove that rank(BA) ≤ min{rank(A), rank(B)}. Solution. Let TA and TB be the unique linear transformations corresponding to A and B, respectively. This question has been solved in lecture; hence, we provide a different approach to the solution. Any vector that y that can be written as a linear combination of the columns of BA as y = BAx can automatically be written as a linear combination of the columns of B by y = B(Ax) = Bx′, for x′ = Ax. Thus col(BA) ⊆ col(B), so rank(BA) ≤ rank(B). Next, let X = AT and Y = BT . Then because rank(XY ) ≤ rank(X) as shown already, taking transposes, we get rank(Y TXT ) ≤ rank(XT ). But because XT = A and Y T = B, we have rank(BA) ≤ rank(A) as desired. It then follows that rank(BA) ≤ min{rank(A), rank(B)} (b) (4 points) What we realized by the previous part is that the rank of BA has some kind of defect from the ranks of A and B. Along those lines, prove the following: rank(BA) = rank(A)− dim(ker(B) ∩ im(A)) Solution 1 (Nice Rank-Nullity). First, rank(BA) = dim(im(TBTA)). By the previous part rank(BA) ≤ rank(A), simply because im(TBTA) ⊆ im(TA). We need to realize the defect now. We rearrange the theorem to say rank(BA) + dim(ker(TB) ∩ im(TA)) = rank(A), and make a clever use of the rank-nullity theorem. Specifically, consider a linear transformation S : im(TA)→ im(TBTA), given by S(x) = Bx for x ∈ im(TA). Then im(S) = im(TBTA) because y = S(x) for some x ∈ im(TA) if and only if y = Bx. However, because x ∈ im(TA), x = Au for some u ∈ Rn. Thus y = BAu, so y ∈ im(TBTA). We conclude then that y ∈ im(S) iff y ∈ im(TBTA), which proves that im(S) = im(TBTA). Next, ker(S) = ker(TB) ∩ im(TA), because ker(S) contains precisely the vectors in im(TA) which get mapped to 0 under B. Now by rank-nullity, dim im(S) + dimker(S) = dim(im(TA)) Then we know that rank(BA) + dim(ker(TB) ∩ im(TA)) = rank(A). Page 3 Math W54 Name: Practice Midterm 2 July 26, 2022 Solution 2 (Extending Basis). This solution is from a more intuitive standpoint, but as we will see, some rigor will be required. We wish to prove that dim im(TBTA) + dim(ker(TB) ∩ im(TA)) = dim im(TA). Notice that any vector in im(A) either goes to zero or goes to something non-zero under TB. In the first case, it becomes a member ker(TB) ∩ im(TA). In the second case, it becomes a member of im(TBTA), excluding zero. Now to say something about dimension, we need to talk about bases. Any basis for im(TA) would come from a basis β = {v1, . . .vp} for ker(TB) ∩ im(TA), and another basis γ corresponding to a basis γ′ for im(TBTA).1 Thus, consider extending the basis β to make a basis for im(TA) as B = {v1, . . .vp,u1, . . .uℓ}. Then dim im(TA) = p+ ℓ, while dim(ker(TB) ∩ im(TA)) = p. If we prove that C = {T (u1), . . . T (uℓ)} is a basis for im(TBTA), then we are done because dim im(TBTA) + dim(ker(TB) ∩ im(TA)) = dim im(TA) = p+ ℓ. This becomes our goal. Linear independence. Suppose a1TB(u1) + · · · aℓTB(uℓ) = 0. By linearity, TB(a1u1 + · · · aℓuℓ) = 0, so a1u1 + · · · aℓuℓ ∈ ker(TB) ∩ im(TA). But because β is a basis for ker(TB) ∩ im(TA), what we have is a1u1 + · · · aℓuℓ = b1v1 + · · ·+ bpvp. Rearranging, a1u1 + · · · aℓuℓ − b1v1 − · · · − bpvp. Since B is a basis for im(TA) however, the set {v1, . . .vp,u1, . . .uℓ} is linearly independent, so a1 = · · · = aℓ = b1 = · · · bp = 0. Thus C is a linearly independent set. Spanning. Take u ∈ im(TA) so that TB(u) ∈ im(TBTA). Then because B is a basis for im(TA), we can write u as u = b1v1 + · · ·+ bpvp + a1u1 + · · · aℓuℓ. Applying TB, we obtain by linearity that TB(u) = b1T (v1) + · · · bpT (vp) + a1T (u1) + · · · aℓT (uℓ) = a1T (u1) + · · · aℓT (uℓ) since v1, . . .vp ∈ ker(TB), hence T (v1), . . . , T (vp) = 0. Thus C indeed spans im(TBTA). (c) (4 points) Finally, something in the other direction: show that rank(BA) + n ≥ rank(A) + rank(B). 1Notice that a very important detail is that we cannot just use the basis for im(TBTA), since this does not live in the same space as im(TA) or ker(TB) ∩ im(TA). This is where the need for rigor comes in. Page 4 Math W54 Name: Practice Midterm 2 July 26, 2022 Solution. We use the rank-nullity theorem. First, subtracting 2n from both sides, we get rank(BA)− n ≥ (rank(A)− n) + (rank(B)− n). Next, multiplying by (−1), n− rank(BA) ≤ (n− rank(A)) + (n− rank(B)). By the rank-nullity theorem, it suffices to prove that2 null(BA) ≤ null(A) + null(B). This is doable. Any vector x ∈ nul(BA) has two possible trajectories: (i) It either goes to 0 under TA, and then TBTA(x) = TB(0) = 0. In this case x ∈ nul(A). (ii) It goes to something non-zero first, but then TA(x) ∈ nul(B), so TBTA(x) = 0. In this case, TA(x) = nul(B), or more specifically TA(x) ∈ nul(B) ∩ im(TA). The idea here is that because nul(BA) is made up of vectors in nul(A) and nul(B) ∩ im(TA), it is missing out on the vectors in nul(B) that are not in im(TA). This is what causes the defect in null(BA), yielding the inequality. We will once again talk about bases to conclude something about dimension. Let β = {v1, . . .vp} be a basis for nul(A). Since nul(A) ⊆ nul(BA), we can extend β to form a basis B = {v1, . . .vp,u1, . . .uℓ} of nul(BA). Thus null(BA) = p+ ℓ, while null(A) = p. We claim that {TA(u1), . . . TA(uℓ)}, vectors in nul(B) ∩ im(TA), form a linearly independent set. If a1TA(u1) + · · · aℓTA(uℓ) = 0, then a1u1 + · · · aℓuℓ ∈ nul(A) by linearity. But since {u1, . . .uℓ} are part of the independent set B, it follows that a1 = · · · = aℓ = 0. Now because u1, . . .uℓ ∈ nul(BA), we know that TBTA(u1), . . . TBTA(uℓ) = 0. Thus TA(u1), . . . TA(uℓ) ∈ nul(B). Thus nul(B) contains a linearly independent set of size ℓ, so any basis it has must be of size at-least ℓ. Thus null(B) ≥ ℓ, giving nul(A) + nul(B) ≥ p+ ℓ = nul(BA) as desired. Looking back, parts (b) and (c) are hard enough to form their own separate questions on an actual midterm. 2Notation thing: we are using nul(A) for the null-space of A, while null(A) = dimnul(A) (called the nullity). Page 5 Math W54 Name: Practice Midterm 2 July 26, 2022 3. (10 points) Given similar matrices A = 1 2 34 5 6 7 8 9 and B = 5 4 62 1 3 8 7 9 . Find a basis in R3 where the matrix for A is B. Solution 1. Observe that we get B by swapping the first two rows and the first two columns of A. In other words, B with its first two rows swapped = A with its first two columns swapped. To swap the first two rows of B, we can multiply the elementary invertible matrix: P = 0 1 01 0 0 0 0 1 on the left of B. Now if we multiply this P on the right of A, it will swap the first two columns of A. So we have: PB = 2 1 35 4 6 8 7 9 = AP So B = P−1AP , and the columns of P is the basis we want. Solution 2. We know B = P−1AP for some invertible matrix P and we want to find the columns of P . Let P = a b cd e f g h i We have PB = AP , or:a b cd e f g h i 5 4 62 1 3 8 7 9 = 1 2 34 5 6 7 8 9 a b cd e f g h i This gives us a system of 9 equations in 9 variables, so we can solve for a, b, c, d, e, f, g, h, i. The basis will be the three column vectors of P . Page 6 Math W54 Name: Practice Midterm 2 July 26, 2022 4. Let P3 denote the vector space of polynomials of degree at most 3, and consider the linear map T : P3 → P3 defined by T (p(t)) = p′(t) + p(0)t3. Calculate the eigenvalues of T . Proof. Observe that T (1) = 0 + t3, T (t) = 1 + 0, T (t2) = 2t+ 0, and T (t3) = 3t2 + 0. Therefore, using the standard coordinates for P3, we see that T is represented by the matrix