# Linear Algebra : Matrix-Matrix Product

## Example Questions

### Example Question #51 : Matrix Matrix Product

True or false: A matrix whose determinant is neither 0 nor 1 cannot be an idempotent matrix.

True

False

True

Explanation:

is an idempotent matrix, by definition, if . Since the determinant of the product of two matrices is equal to the product of their determinants, it follows that

and, since

.

By transitivity,

.

The only two numbers equal to their own squares are 0 and 1, so

or .

This makes the statement true.

### Example Question #52 : Matrix Matrix Product

.

Calculate .

is undefined.

Explanation:

, the conjugate transpose of , can be found by first taking the transpose of :

so

then changing each element to its complex conjugate:

Find the product  by multiplying the rows of  by the columns of ; that is, add the product of the terms in corresponding positions:

### Example Question #53 : Matrix Matrix Product

Calculate .

is undefined.

Explanation:

, the transpose of , is the result of switching the rows of  with the columns.

so

Find the product  by multiplying the rows of  by the columns of ; that is, add the product of the terms in corresponding positions:

### Example Question #54 : Matrix Matrix Product

Always, sometimes, or never: .

Give the answer for both square and nonsquare matrices.

Square: Always

Nonsquare: Sometimes

Square: Never

Nonsquare: Never

Square: Always

Nonsquare: Never

Square: Sometimes

Nonsquare: Sometimes

Square: Sometimes

Nonsquare: Never

Square: Sometimes

Nonsquare: Never

Explanation:

The statement cannot be true for nonsquare matrices. For  to be defined, the number of columns in  must be equal to the number of rows in ; for  to be defined, the reverse must hold. It follows that  and  must be  and  matrices, respectively.

The product of two matrices has the same number of rows as the former matrix and the same number of columns as the latter. Therefore,  is an  matrix and  is an  matrix. If , then  and  do not even have the same dimensions. Therefore,  is always false for nonsquare matrices.

We now show that  for some, but not all square matrices. If, it easily follows that , since . Now let

and .

The products are

and

. Since at least one case exists in which  and at least one case exists in which , the statement  is sometimes true for square matrices.

### Example Question #51 : Matrices

It is recommended that you use a calculator with matrix arithmetic capability for this problem.

The above diagram shows a board for a game of chance. A player moves according to the flip of a fair coin, depending on his current location. For example, if he is on the green square, he will move to the orange square if the coin comes up heads, and to the pink square if it comes up tails.

If, after eight moves, a player ends up on the green square, he wins. Which square should he start out on in order to maximize his probability of winning?

Green

Blue

Orange

Pink

Pink

Explanation:

Since the coin is fair, each outcome will come up with probability 0.5. Therefore, a player on orange end up on orange or pink with 0.5 probability each; a player on pink will end up on orange or blue with probability 0.5 each;  a player on blue will end up on pink or green with probability 0.5 each; and a player on green will end up on orange or pink with probability 0.5 each. The stochastic matrix that models this is

.

The matrix that models the probabilities that the player will end up on a given square after eight moves, given that he starts on a particular square, is , which through calculation is

Since he wants to land on green after eight turns, examine the fourth row. The greatest probability of ending up on green, albeit by very little, appears in the second row. Therefore, to maximize his chances of ending up on green, the player should start on pink.

### Example Question #741 : Linear Algebra

.

Calculate .

is undefined.

Explanation:

, the transpose of , is the result of switching the rows of  with the columns.

so

Find the product  by multiplying the rows of  by the columns of ; that is, add the product of the terms in corresponding positions:

### Example Question #57 : Matrix Matrix Product

.

Calculate .

is undefined.

Explanation:

, the conjugate transpose of , can be found by first taking the transpose of :

so

then changing each element to its complex conjugate:

Find the product  by multiplying the rows of  by the columns of ; that is, add the product of the terms in corresponding positions:

### Example Question #52 : Matrices

is a  nonsingular matrix;  is a  matrix;  is a  singular matrix.

Which of the following is defined?

Explanation:

can be eliminated as a choice;  is not a square matrix, so its inverse, , is undefined.

can be eliminated as a choice, since it is given that  is singular - that is,  does not exist.

For the product of three matrices to be defined, they must be, in order, an  matrix, an  matrix, and a  matrix. We examine the three remaining choices.

:

is a  matrix, so  is as well;  is a  matrix, so its transpose  is a  matrix;  is a   matrix. These are incompatible, so  is undefined.

is a  matrix;  is a  matrix;  is a  matrix; These are also incompatible, so  is undefined.

:

is a  matrix;  is a  matrix, as stated before;  is a  matrix. These are compatible, so  is defined. This is the correct choice.

### Example Question #59 : Matrix Matrix Product

Some children are playing a game that employs the above board and a pair of fair six-sided dice.

According to the rules of the game, if you are on one of the six spaces of the hexagonal board at left:

1) If you roll a 6, you go to the gray "Time Out!" square.

2) If you roll any other number, you move that many squares clockwise.

If you are on the gray "Time Out!" square:

1) If you roll anything but doubles, you stay on the gray square.

2) If you roll doubles, you re-enter the hexagonal board on the space indicated by the particular double - for example, if you roll 1-1, you re-enter the board on the red space; 2-2, the blue space; and so forth.

Construct a stochastic matrix to model the probability of ending up on each space on any given turn. Let the first row/column represent the red space, the second row/column represent the blue space, and so forth clockwise, and let the seventh row/column represent the gray square.

Explanation:

If a player is on the red space, he will end up on each of the other spaces with the indicated rolls:

Red: 12; 1 roll out of 36  - probability .

Blue: 7; 6 rolls out of 36 possible - probability .

Green: 2 (1 roll) or 8 (5 rolls); 6 rolls out of 36 - probability .

Orange: 3 (2 rolls) or 9 (4 rolls); 6 rolls out of 36 - probability .

Lavender: 4 (3 rolls) or 10 (3 rolls); 6 rolls out of 36 - probability .

Brown: 5 (4 rolls) or 11 (2 rolls); 6 rolls out of 36 - probability .

Gray ("Time out"): 6; 5 rolls out of 36  - probability .

The  stochastic matrix with the first column only filled in is

The rules of the rolls are analogous for each of the other spaces, keeping the probabilities, relatively speaking, the same - the probability of staying on the same space is , that of landing on a given other space on the hexagon is , and the probability of ending up on the gray square is . The next five columns can be filled in as follows:

A player on the gray square can only end up any given space on the hexagon by rolling one specific pair of doubles, making each of these probabilities equal to . The other 30 rolls will keep him on the gray square, so there is a  probability he will stay on the gray square on that turn. Therefore, the final column can be filled in as follows:

This matrix is the correct choice.

### Example Question #51 : Matrix Matrix Product

Some children are playing a game that employs the above board and a pair of fair six-sided dice.

According to the rules of the game:

If you are on one of the six spaces of the hexagonal board at left, you roll one die. If roll a 6, you go to the gray square marked "Time Out!"; otherwise, you move clockwise the number of squares indicated.

If you are on the gray square marked "Time Out!", you roll both dice. If you roll anything but doubles, you stay on the gray square.  If you roll doubles, you re-enter the hexagonal board on the space indicated by the particular double - for example, if you roll 1-1, you re-enter the board on the red space; 2-2, the blue space; and so forth.

Construct a stochastic matrix to model the probability of ending up on each space on any given turn. Let the first row/column represent the red space, the second row/column represent the blue space, and so forth clockwise, and let the seventh row/column represent the gray square.