# Linear Algebra/Changing Map Representations/Solutions

## Solutions

edit*This exercise is recommended for all readers.*

- Problem 1

Decide if these matrices are matrix equivalent.

- ,
- ,
- ,

- Answer

- Yes, each has rank two.
- Yes, they have the same rank.
- No, they have different ranks.

*This exercise is recommended for all readers.*

- Problem 2

Find the canonical representative of the matrix-equivalence class of each matrix.

- Answer

We need only decide what the rank of each is.

- Problem 3

Suppose that, with respect to

the transformation is represented by this matrix.

Use change of basis matrices to represent with respect to each pair.

- ,
- ,

- Answer

Recall the diagram and the formula.

- These two
- These two

*This exercise is recommended for all readers.*

- Problem 4

What sizes are and in the equation ?

- Answer

Where and are , the matrix is while is .

*This exercise is recommended for all readers.*

- Problem 5

Use Theorem 2.6 to show that a square matrix is nonsingular if and only if it is equivalent to an identity matrix.

- Answer

Any matrix is nonsingular if and only if it has rank , that is, by Theorem 2.6, if and only if it is matrix equivalent to the matrix whose diagonal is all ones.

*This exercise is recommended for all readers.*

- Problem 6

Show that, where is a nonsingular square matrix, if and are nonsingular square matrices such that then .

- Answer

If then , so , and so .

*This exercise is recommended for all readers.*

- Problem 7

Why does Theorem 2.6 not show that every matrix is diagonalizable (see Example 2.2)?

- Answer

By the definition following Example 2.2, a matrix is diagonalizable if it represents a transformation with the property that there is some basis such that is a diagonal matrix— the starting and ending bases must be equal. But Theorem 2.6 says only that there are and such that we can change to a representation and get a diagonal matrix. We have no reason to suspect that we could pick the two and so that they are equal.

- Problem 8

Must matrix equivalent matrices have matrix equivalent transposes?

- Answer

Yes. Row rank equals column rank, so the rank of the transpose equals the rank of the matrix. Same-sized matrices with equal ranks are matrix equivalent.

*This exercise is recommended for all readers.*

- Problem 10

Show that matrix-equivalence is an equivalence relation.

- Answer

For reflexivity, to show that any matrix is matrix equivalent to itself, take and to be identity matrices. For symmetry, if then (inverses exist because and are nonsingular). Finally, for transitivity, assume that and that . Then substitution gives . A product of nonsingular matrices is nonsingular (we've shown that the product of invertible matrices is invertible; in fact, we've shown how to calculate the inverse) and so is therefore matrix equivalent to .

*This exercise is recommended for all readers.*

- Problem 11

Show that a zero matrix is alone in its matrix equivalence class. Are there other matrices like that?

- Answer

By Theorem 2.6, a zero matrix is alone in its class because it is the only of rank zero. No other matrix is alone in its class; any nonzero scalar product of a matrix has the same rank as that matrix.

- Problem 12

What are the matrix equivalence classes of matrices of transformations on ? ?

- Answer

There are two matrix-equivalence classes of matrices— those of rank zero and those of rank one. The matrices fall into four matrix equivalence classes.

- Problem 13

How many matrix equivalence classes are there?

- Answer

For matrices there are classes for each possible rank: where is the minimum of and there are classes for the matrices of rank , , ..., . That's classes. (Of course, totaling over all sizes of matrices we get infinitely many classes.)

- Problem 14

Are matrix equivalence classes closed under scalar multiplication? Addition?

- Answer

They are closed under nonzero scalar multiplication, since a nonzero scalar multiple of a matrix has the same rank as does the matrix. They are not closed under addition, for instance, has rank zero.

- Problem 15

Let represented by with respect to .

- Find in this specific case.
- Describe in the general case where .

- Answer

- Problem 16

- Let have bases and and suppose that has the basis . Where , find the formula that computes from .
- Repeat the prior question with one basis for and two bases for .

- Answer

- Problem 17

- If two matrices are matrix-equivalent and invertible, must their inverses be matrix-equivalent?
- If two matrices have matrix-equivalent inverses, must the two be matrix-equivalent?
- If two matrices are square and matrix-equivalent, must their squares be matrix-equivalent?
- If two matrices are square and have matrix-equivalent squares, must they be matrix-equivalent?

- Answer

- Here is the arrow diagram, and a version of that diagram
for inverse functions.
Yes, the inverses of the matrices represent the inverses of the maps. That is, we can move from the lower right to the lower left by moving up, then left, then down. In other words, where (and invertible) and are invertible then .

- Yes; this is the prior part repeated in different terms.
- No, we need another assumption: if represents
with respect to the same starting as ending bases ,
for some then represents
.
As a specific example,
these two matrices are both rank one and so they are
matrix equivalent
- No.
These two are not matrix equivalent but have matrix equivalent
squares.

*This exercise is recommended for all readers.*

- Problem 18

Square matrices are **similar** if they represent the same
transformation, but each with respect to the same ending as starting
basis.
That is, is similar to .

- Give a definition of matrix similarity like that of Definition 2.3.
- Prove that similar matrices are matrix equivalent.
- Show that similarity is an equivalence relation.
- Show that if is similar to then
is similar to , the cubes are similar, etc.
*Contrast with the prior exercise.* - Prove that there are matrix equivalent matrices that are not similar.

- Answer

- The definition is suggested by the appropriate
arrow diagram.
Call matrices
**similar**if there is a nonsingular matrix such that . - Take to be and take to be .
*This is as in Problem 10.*Reflexivity is obvious: . Symmetry is also easy: implies that (multiply the first equation from the right by and from the left by ). For transitivity, assume that and that . Then and we are finished on noting that is an invertible matrix with inverse .- Assume that . For the squares: . Higher powers follow by induction.
- These two are matrix equivalent but their squares are not
matrix equivalent.