MATH2022 Linear and Abstract Algebra Semester 1 Exercises for Week 10
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MATH2022 Linear and Abstract Algebra
Semester 1
Exercises for Week 10
Important Ideas and Useful Facts:
(i) Matrix exponentials: If M is a real square matrix then we may form the matrix exponential
eM = I + M + + + ... .
It is a theorem that the series always converges. If M is a diagonal n × n matrix with diagonal entries λ 1 , . . . ,λn then eM is also diagonal with diagonal entries eλ 1 , . . . ,eλn . If
A, B and P are real square matrices of the same size, P invertible, and B = P −1AP then
eB = P −1eA P .
If A and B commute, that is, AB = BA, then eA+B = eA eB .
(ii) Solving systems of differential equations: Suppose that we have n differentiable functions
x1 = x1 (t),x2 = x2 (t), . . . ,xn = xn (t) of a real variable t that satisfy the following system of differential equations with constant coefficients:
x = a11 x1 + a12 x2 + ··· + a1nxn
x = a21 x1 + a22 x2 + ··· + a2nxn
x = an1x1 + an2x2 + ··· + ann xn
l x(x)2(1) 」 l x(x) 」 l 」
Put x = , x\ = and A = , so that the system may
. . .
「 xn 「 x 「 an1 an2 . . . ann
be expressed in matrix form x\ = Ax. The solution to this system is
x = etA c
where c = x(0) is a column vector of constants.
(iii) Linear transformations (general case): Let V and W be vector spaces over a field F . A
function T : V → W is called a linear transformation if T respects vector addition and scalar multiplication, that is, for all v, w ∈ V and λ ∈ F ,
T(v + w) = T(v) + T(w) and T(λv) = λT(v) ,
or, equivalently, T preserves linear combinations, that is for all v1 , v2 ∈ V and λ 1 ,λ2 ∈ F ,
T(λ1v1 + λ2v2 ) = λ 1T(v1 ) + λ2T(v2 ) .
If V = W then T is called a linear operator. If T is bijective (one-one and onto) then T is called a vector space isomorphism. The composite of linear transformations, when defined, is also a linear transformation.
(iv) Matrix of a linear transformation with respect to choice of bases: Let T : V → W be a linear
transformation, and let B = {b1 , . . . , bn } and D = {d1 , . . . , dm } be ordered bases for V and W respectively. Define the matrix of T with respect to B and D to be
[T]D(B) = [ [T(b1 )]D ... [T(bn )]D ] ,
by which we mean that we write down, in order, columns of coordinates, in W with respect to D, of the images under T of successive basis elements from B . Note that [T]D(B) is an m × n matrix. It follows from the definitions that, for all v ∈ V ,
[T(v)]D = [T]D(B) [v]B ,
enabling the effect of the linear transformation T to be described in terms of matrix mul- tiplication between coordinates of vectors. If S : U → V is another linear transformation, where A is an ordered basis for U, so that T 。S : U → W is also a linear transformation, then
[T 。S]D(A) = [T]D(B)[S]B(A) .
(v) The identity linear operator: Given any vector space V the mapping id = idV : V → V where id(v) = v, fixing all vectors in V , is called the identity linear transformation or identity operator. If V is n-dimensional and B is any basis for V then [id]B(B) = In , the n × n identity matrix. If T : V → W is a linear transformations then
T 。idV = T and idW 。T = T .
Further, if T is a vector space isomorphism, so that T is invertible and T −1 : W → V , then
T −1 。T = idV and T 。T −1 = idW .
(vi) Change of basis matrix: Let B and D be any bases for an n-dimensional vector space V .
The matrix [id]D(B) is called a change of basis matrix and has the effect of converting coor- dinates of vectors with respect to B into coordinates with respect to D, in the following sense, for any vector v ∈ V :
[id]D(B) [v]B = [v]D .
Furthermore, the change of basis matrices [id]D(B) and [id]B(D) are mutually inverse, that is, [id]D(B) [id]B(D) = [id]B(D) [id]D(B) = In .
Tutorial Exercises:
1. Find the exponential matrix etA where A is each of the following matrices:
(a) ] (b) [1(1) 1(1) ] (c) [2(1) 2(3) ] (d) ]
2. Solve the following systems of differential equations, where x = x(t) and y = y(t) are differentiable functions of a real variable t, with the same initial conditions
x(0) = 1 and y(0) = 2
in each case:
(a) |
x\ = −x y\ = 2y |
(b) |
x\ = x + y y\ = x + y |
(c) |
x\ = x + 3y y\ = 2x + 2y |
(d) |
x\ = 5x − 6y y\ = 3x − 4y |
3. Let B = {(1, 0), (0, 1)} be the standard basis for R2 . Put D = {(1, 1), (−1, 0)} .
Explain why D is a basis for R2 and then write down the following matrices: A = [id]B(B) , C = [id]D(D) and E = [id]B(D) .
Now find E −1 in the usual way and check that indeed
E −1 = [ [(1, 0)]D [(0, 1)]D ] = [id]D(B) .
4. Let f,g : R2 → R2 be linear transformations given by the following rules: f(x,y) = (x + 2y,3x − 4y) and g(x,y) = (3x − y,2y) .
(a) Find each of the following, by direct calculation, where B and D are the bases for
R2 in the previous exercise:
[f]B(B) , [f]D(D) , [g]B(B) , [g]D(D) .
(If you have done this correctly, you should have produced a diagonal matrix repre- sentation for g .)
(b) Check, as the theory predicts, that the following equations hold:
[f]D(D) = [id]D(B)[f]B(B)[id]B(D) and [g]D(D) = [id]D(B)[g]B(B)[id]B(D) .
(c)* Find rules for linear operators h,k : R2 → R2 such that [h]B(B) = [f]B(D) and [k]B(B) = [f]D(B) .
5.* Working over R, let B = {1,x,x2 } be the standard basis for the vector space P2 of
polynomials of degree at most 2. Put
D = {1 + x2 ,x + 2x2 , 1 + 2x + 3x2 } .
Explain why D is a basis for P2 and then write down the matrix E = [id]B(D) . Now find E −1 in the usual way and check that indeed
E −1 = [ [1]D [x]D [x2]D ] = [id]D(B) .
Further Exercises:
6. Let B = {(1, 0, 0), (0, 1, 0), (0, 0, 1)} be the standard basis for R3 . Put D = {(1, 0, 1), (1, 1, 0), (1, 1, 1)} .
Explain why D is a basis for R3 and then write down the matrix E = [id]B(D) . Now find E −1 in the usual way and check that indeed
E −1 = [ [(1, 0, 0)]D [(0, 1, 0)]D [(0, 0, 1)]D ] = [id]D(B) .
7. Find the exponential matrix etA where A is each of the following matrices:
(a) 「(l) − (b) 「(l) − (c) 「(l) −
8. Solve the following systems of differential equations, where x = x(t), y = y(t) and z = z(t) are differentiable functions of a real variable t, with the same initial conditions
x(0) = −1 , y(0) = −4 and z(0) = 2
in each case:
x\ = −x
(a) y\ = 2y (b)
z\ = 3z
x\ = x + y + 2z
(c) y\ = −y
z\ = 2x + y + z
9. Consider the real matrix M = ].
x\ = y
y\ = x + z\ = x + y
− z
z
(a) Write down the rule for the linear transformation f : R3 → R2 such that the matrix
of f with respect to the standard bases is M .
(b) Explain briefly why B = {(1, 1, 1), (1, 1, 0), (1, 0, 0)} and D = {(1, 3), (2, 5)} are
bases for R3 and R2 respectively.
(c)* Find the matrix [f]D(B) of f with respect to B and D .
10.∗ Let D be the differential operator that takes a differentiable function to its derivative.
Explain why each of the following sets is a basis of the subspace of RR that it generates:
B1 = {1,x,x2 ,x3 }, B2 = {sinx,cosx}, B3 = {ex ,e2x,xe2x} .
Each of these subspaces consists of differentiable functions on which D acts as an operator. Find [D]B(B)i(i) for i = 1, 2, 3 and calculate the rank and nullity of D in each case.
2023-05-23