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# Compute the eigenvalues and corresponding eigenvectors of the matrix A

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Cornelis VAN DER MEE, Spring 2008, Math 3330, Sample Exam 4 Name: . . . Grade: . . . Rank: . . . . To receive full credit, show all of your work. Neither calculators nor computers are allowed.

1. Compute the eigenvalues and corresponding eigenvectors of the matrix A = 3 −2

15 −10

 .

Use this information to diagonalize the matrix A if possible. Other- wise indicate why diagonalization is not possible. Solution: First we compute the characteristic polynomial

det(λI2− A) = detλ − 3 2

−15 λ + 10



= (λ − 3)(λ + 10) + 30 = λ(λ + 7).

The eigenvalues of A are its zeros 0 and −7. The eigenspace correspond- ing to the eigenvalue λ = 0 (which is Ker A) consists of the multiples of the vector 2

3



. The eigenspace corresponding to the eigenvalue

−7 (which is Ker(A + 7I2) = Ker10 −2 15 −3



) consists of the multiples of the vector 1

5



. Since the eigenvalues of A are distinct, we can diagonalize A. In fact,

A2 1 3 5



| {z }

=S

=2 1 3 5



| {z }

=S

0 0 0 −7

 ,

where S is the (invertible) diagonalizing transformation.

2. Find a 2 × 2 matrix A such that 1 2



and 2 3



are eigenvectors of A, with eigenvalues −2 and 3, respectively. Solution: We obviously have

A1 2



= −21 2



=−2

−4



, A2 3



= 32 3



=6 9

 ,

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or in other words

A1 2 2 3



=−2 6

−4 9

 . Hence,

A =−2 6

−4 9

 1 2 2 3

−1

=−2 6

−4 9

 −3 2 2 −1



=18 −10 30 −17

 .

3. Find the solution of the discrete dynamical system x(n + 1) = Ax(n), n = 0, 1, 2, 3, . . . , where

A =2 3 0 1



, x(0) =0 1

 .

Solution: The solution is, in abstract form, x(n) = Anx(0), where n = 0, 1, 2, . . .. To find a more amenable solution we diagonalize A, AS = Sdiag(λ1, λ2), put y(n) = Sx(n), and write the dynamical system in the simplified form

y(n+1) = Sx(n+1) = SAx(n) = diag(λ1, λ2)Sx(n) = diag(λ1, λ2)y(n), which is easy to solve. Since A is upper triangular, its eigenvalues are its diagonal elements λ1 = 2 and λ2 = 1, while it is easily seen that A1

0



= 21 0



. To find the other eigenvector (which must be linearly independent and hence must have a nonzero second entry), we try

Ax 1



=x 1

 ,

which implies 2x + 3 = x and 1 = 1. Thus x = −3 and −3 1

 is the eigenvector corresponding to the eigenvalue 1. We now line up the two eigenvectors as column of a 2 × 2 matrix to get

S =1 −3

0 1

 .

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Since y(0) = Sx(0) =−3 1



, we get

y(n) =2 0 0 1

n

−3 1



=−3 · 2n 1

 . Hence,

x(n) = S−1y(n) =1 3 0 1

 −3 · 2n 1



=3[1 − 2n] 1

 .

4. Find all eigenvalues (real and complex) of the matrix

A =

0 1 0

0 0 1

4 −4 1

.

Why or why not is it possible to diagonalize the matrix A? Solution:

Let us compute the characteristic polynomial of A:

det(λI3− A) = det

λ −1 0

0 λ −1

−4 4 λ − 1

= λ detλ −1 4 λ − 1



+ 0 −1

−4 λ − 1



= λ{λ(λ − 1) + 4} − 4 = λ3− λ2+ 4λ − 4

= (λ2+ 4)(λ − 1).

Since the eigenvalues [1, 2i, and −2i] are distinct, it is possible to diagonalize A.

5. Compute the eigenvalues and corresponding eigenvectors of the matrix

A =

0 0 0 1

0 3 −3 1

0 1 0 0

0 0 1 0

 .

Use this information to diagonalize the matrix A if possible. Otherwise indicate why diagonalization is not possible. Solution: Let us compute

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the characteristic polynomial

det(λI4− A) = det

λ 0 0 −1

0 λ − 3 3 −1

0 −1 λ 0

0 0 −1 λ

= λ det

λ − 3 3 −1

−1 λ 0

0 −1 λ

= λ{λ2(λ − 3) − 1 + 3λ} = λ(λ − 1)3.

Thus the eigenvalues are 0 and 1, where 1 has algebraic multiplicity 3.

Since the first column of A has only zeros, the eigenvector corresponding to the zero eigenvalue is (1, 0, 0, 0)T. Now observe that

(I4− A)

 x1 x2

x3 x4

=

1 0 0 −1

0 −2 3 −1

0 −1 1 0

0 0 −1 1

 x1 x2

x3 x4

=

x1− x4

−2x2+ 3x3− x4

−x2+ x3

−x3+ x4

 ,

which can only be the zero vector if x1 = x2 = x3 = x4 = 1. Thus the eigenspace corresponding to the eigenvalue 1 is one-dimensional and hence 1 is an eigenvalue of geometric multiplicity 1. Since algebraic and geometric multiplicities differ and hence there is no basis of R4 consisting of eigenvectors of A, the matrix A cannot be diagonalizable.

6. Compute the eigenvalues (real and complex) and corresponding eigen- vectors of the matrix

A =

0 0 0 1 1 0 0 0 0 1 0 0 0 0 1 0

 .

Explain why your result is in full agreement with the values of Tr(A)

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and det(A). Solution: Let us compute the characteristic polynomial

det(λI4− A) = det

λ 0 0 −1

−1 λ 0 0

0 −1 λ 0

0 0 −1 λ

= λ det

λ 0 0

−1 λ 0

0 −1 λ

| {z }

lower triangular

+

0 0 −1

−1 λ 0

0 −1 λ

= λ4− 1 = (λ − 1)(λ + 1)(λ − i)(λ + i).

Thus the eigenvalues are 1, −1, i, and −i. Their sum is Tr A = 0.

Their product is det(A) = −1.

7. Why is 45 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 an eigenvalue corresponding to any 9 × 9 sudoku matrix? What is the corresponding eigenvector?

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