MTH107 Advanced Linear Algebra 2022/23
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MTH107 Advanced Linear Algebra
2022/23
Use these problems after you finish reviewing all tutorial problems and quiz problems. In what follows, let F be the field R or C.
1. Consider the vector space Fn,n of n × n matrices with entries in F. Then dimFn,n = n2 .
(a) A matrix A ∈ Fn,n is called symmetric if A = AT (where AT is the transpose of A). Let Symn(F) be the set of all symmetric matrices in Fn,n ,
Symn(F) = {A ∈ Fn,n | A = AT}.
Prove that Symn(F) is a subspace of Fn,n and find its dimension.
(b) A matrix A ∈ Fn,n is called alternating if −A = AT . Let Altn(F) be the set of all alternating matrices in Fn,n ,
Altn(F) = {A ∈ Fn,n | −A = AT}.
Prove that Altn(F) is a subspace of Fn,n and find its dimension.
(c) Prove that Fn,n = Symn(F) ⊕ Altn(F).
2. Define a map T : Fn,n → Fn,n by T(A) = A − AT . Prove that T is linear and find null T and range T .
3. Let U,V be vector spaces over F and S,T ∈ L(U,V). Prove or find a counterexample for the following claims.
(a) null(S + T) = null(S) ∩ null(T).
(b) range(S + T) = range(S) + range(T).
4. Suppose U,V,W are finite-dimensional vector spaces over F and T ∈ L(U,V) and S ∈ L(V,W), so ST ∈ L(U,W).
(a) Prove that dimrange(ST) ≤ dimrange(S).
(b) Prove that dimrange(ST) ≤ dimrange(T).
(c) Suppose ST = 0 ∈ L(U,W). Prove that dimrange(T) + dimrange(S) ≤ dimV .
5. Let V be a finite-dimensional vector space and T ∈ L(V). Prove the following are equivalent. (Prove (a)⇒(b)⇒(c)⇒(a).)
(a) V = null T ⊕ range T .
(b) null T ∩ range T = {0}.
(c) dimrange(T2) = dimrange T .
6. Let V,W be vector spaces with dimV = n > dimW = m and T ∈ L(V,W). Suppose U is a subspace of V with dimU = n − 1 and the restriction T|U ∈ L(U,W) of T to U is surjective. Show that null T ⊈ U .
7. Let Pk(F) be the vector space of polynomials of degree at most k with coefficients in F. Fix a
polynomial f(x) of degree d and define the following map called multiplication by f(x), Tf : Pn(F) → Pn+d(F), Tf(p(x)) := f(x)p(x).
(a) Prove that Tf is a linear map.
(b) Let Bk = {1,x,x2 , . . . ,xk } be the standard basis for Pk(F). Find the matrix representation [Tf] of Tf with respect to standard bases in the case f(x) = x2 + x + 1 and n = 3.
8. Fix a scalar a ∈ F and define a map
p(x) − p(a)
x − a .
(a) Prove that the above defined Da(p(x)) is indeed a polynomial in Pn−1(F). (b) Prove that Da is a linear map.
(c) In the case n = 3, choose suitable bases and find the matrix representation of Da .
9. Fix a matrix A ∈ F2,2 and define a map
adA : F2,2 → F2,2 , adA (X) := AX − XA.
Prove that adA is linear and find its matrix representation with a suitable basis.
10. Let Rn be the n-dimensional Euclidean space with standard basis {e1 = (1, 0, . . . , 0), . . . ,en = (0, . . . , 0, 1)} . Let U be the subspace
U := {(x1 , . . . ,xn) ∈ Rn | x1 + ··· + xn = 0} .
(a) Find a basis for U⊥ .
(b) Write v1 := e1 − e2 ,v2 := e2 − e3 , . . . ,vn−1 := en−1 − en . Prove that {v1 , . . . ,vn−1} is a
basis for U .
(c) Apply Gram-Schmidt procedure to {v1 , . . . ,vn−1} to find an orthonormal basis {b1 , . . . ,bn−1} for U .
(d) Given a vector x = (x1 , . . . ,xn) ∈ Rn, find its orthogonal projection PU(x) onto U and orthogonal projection PU⊥ (x) onto U⊥ .
11. Let V be the real inner product space of smooth functions defined on interval [−π,π], with inner product ⟨f,g⟩ = lπf(x)g(x)dx. Let U be the subspace of V spanned by 1 (the constant function), sin x and cos x. Find the best approximation u ∈ U of ex ∈ V .
12. Let V be a vector space over F with a basis B = {v1 , . . . ,vn} . Suppose T ∈ L(V) is defined by T(vj) = v1 + ··· + vn for j = 1, . . . ,n.
(a) Find all eigenvalues of T .
(b) Find a basis for each eigenspace of T .
(c) Is T diagonalizable? If yes, find a basis D such that [T]D(D) is diagonal; if no, state the reason.
13. Let V be a vector space over F with a basis B = {v1 , . . . ,vn}. Suppose T ∈ L(V) is defined by T(vj) = vn+1−j for j = 1, . . . ,n.
(a) Find all eigenvalues of T .
(b) Find a basis for each eigenspace of T .
(c) Is T diagonalizable? If yes, find a basis D such that [T]D(D) is diagonal; if no, state the reason.
14. Let V be a vector space over C with a basis B = {v1 , . . . ,vn}. Suppose T ∈ L(V) is defined by T(vj) = v1 + ··· + vj for j = 1, . . . ,n. Find a basis D such that [T]D(D) is a Jordan form of T .
15. Let V be a vector space over C with a basis B = {v1 , . . . ,v2023 }. Suppose T ∈ L(V) is defined by T(v1 ) = 0, T(v2 ) = v1 ,T(v3 ) = v2 , . . . ,T(v2023 ) = v2022 .
(a) Find the Jordan form and the minimal polynomial of T .
(b) Find the Jordan form and the minimal polynomial of T2022 .
16. Find as many matrices A1,A2 , . . . ,Am ∈ C4,4 satisfying the following conditions as possible:
(a) Ai has only one eigenvalue 1 for i = 1, . . . ,m.
(b) If i j, then Ai and Aj are not similar.
Write down also the characteristic polynomial and the minimal polynomial of each Ai . What is the maximum of m? (Hint: let Ai be of its Jordan form).
2023-01-13