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Prelude

- A pattern of activation in a NN is a vector
- A set of connection weights between units is a matrix
- Vectors and matrices have well-understood mathematical and geometric properties
- Very useful for understanding the properties of NNs

Outline

- The Players: Scalars, Vectors and Matrices
- Vectors, matrices and neural nets
- Geometric Analysis of Vectors
- Multiplying Vectors by Scalars
- Multiplying Vectors by Vectors
- The inner product (produces a scalar)
- The outer product (produces a matrix)
- Multiplying Vectors by Matrices
- Multiplying Matrices by Matrices

Scalars, Vectors and Matrices

- Scalar: A single number (integer or real)
- Vector: An ordered list of scalars

[ 1 2 3 4 5 ] [ 0.4 1.2 0.07 8.4 12.3 ] [ 12 10 ] [ 2 ]

Scalars, Vectors and Matrices

- Scalar: A single number (integer or real)
- Vector: An ordered list of scalars

[ 1 2 3 4 5 ] [ 0.4 1.2 0.07 8.4 12.3 ] [ 12 10 ] [ 2 ]

[ 12 10 ] ≠ [ 10 12 ]

Scalars, Vectors and Matrices

- Scalar: A single number (integer or real)
- Vector: An ordered list of scalars

[ 1 2 3 4 5 ] [ 0.4 1.2 0.07 8.4 12.3 ] [ 12 10 ] [ 2 ]

Row vectors

2

3

4

5

1.5

0.3

6.2

12.0

17.1

Scalars, Vectors and Matrices- Scalar: A single number (integer or real)
- Vector: An ordered list of scalars

[ 1 2 3 4 5 ] [ 0.4 1.2 0.07 8.4 12.3 ] [ 12 10 ] [ 2 ]

Row vectors

Column Vectors

Scalars, Vectors and Matrices

Scalar: A single number (integer or real)

Vector: An ordered list of scalars

Matrix: An ordered list of vectors:

1 2 6 1 7 8

2 5 9 0 0 3

3 1 5 7 6 3

2 7 9 3 3 1

Scalars, Vectors and Matrices

Scalar: A single number (integer or real)

Vector: An ordered list of scalars

Matrix: An ordered list of vectors:

1 2 6 1 7 8

2 5 9 0 0 3

3 1 5 7 6 3

2 7 9 3 3 1

Row vectors

Scalars, Vectors and Matrices

Scalar: A single number (integer or real)

Vector: An ordered list of scalars

Matrix: An ordered list of vectors:

1 2 6 1 7 8

2 5 9 0 0 3

3 1 5 7 6 3

2 7 9 3 3 1

Column vectors

Scalars, Vectors and Matrices

Scalar: A single number (integer or real)

Vector: An ordered list of scalars

Matrix: An ordered list of vectors:

1 2 6 1 7 8

2 5 9 0 0 3

3 1 5 7 6 3

2 7 9 3 3 1

Matrices are indexed (row, column)

M =

Scalars, Vectors and Matrices

Scalar: A single number (integer or real)

Vector: An ordered list of scalars

Matrix: An ordered list of vectors:

1 2 6 1 7 8

2 5 9 0 0 3

3 1 5 7 6 3

2 7 9 3 3 1

Matrices are indexed (row, column)

M(1,3) = 6 (row 1, column 3)

M =

Scalars, Vectors and Matrices

Scalar: A single number (integer or real)

Vector: An ordered list of scalars

Matrix: An ordered list of vectors:

1 2 6 1 7 8

2 5 9 0 0 3

3 1 5 7 6 3

2 7 9 3 3 1

Matrices are indexed (row, column)

M(1,3) = 6 (row 1, column 3)

M(3,1) = 3 (row 3, column 1)

M =

Variable Naming Conventions

- Scalars: Lowercase, italics

x, y, z…

- Vectors: Lowercase, bold

u, v, w…

- Matrices: Uppercase, bold

M, N, O …

- Constants: Greek

, , , , …

Transposing Vectors

If u is a row vector…

u = [ 1 2 3 4 5 ]

…then u’ (“u-transpose”) is a column vector

1

2

3

4

5

… and vice-versa.

u’ =

Transposing Vectors

If u is a row vector…

u = [ 1 2 3 4 5 ]

…then u’ (“u-transpose”) is a column vector

1

2

3

4

5

… and vice-versa.

Why in the world

would I care??

u’ =

You

Transposing Vectors

If u is a row vector…

u = [ 1 2 3 4 5 ]

…then u’ (“u-transpose”) is a column vector

1

2

3

4

5

… and vice-versa.

Answer: It’ll matter when we come to vector multiplication.

u’ =

Transposing Vectors

If u is a row vector…

u = [ 1 2 3 4 5 ]

…then u’ (“u-transpose”) is a column vector

1

2

3

4

5

… and vice-versa.

OK.

u’ =

Vectors, Matrices & Neural Nets

i1

i2

Output units, i

Connection weights, wij

w23

w11

w13

w12

w22

w21

j1

j2

j3

Input units, j

Vectors, Matrices & Neural Nets

i1

i2

Output units, i

Connection weights, wij

w23

w11

w13

w12

w22

w21

0.2

0.9

0.5

Input units, j

The activations of the input nodes can be represented as a 3-dimensional vector:

j = [ 0.2 0.9 0.5 ]

Vectors, Matrices & Neural Nets

1.0

0.0

Output units, i

Connection weights, wij

w23

w11

w13

w12

w22

w21

j1

j2

j3

Input units, j

The activations of the output nodes can be represented as a 2-dimensional vector:

i = [ 1.0 0.0 ]

Vectors, Matrices & Neural Nets

i1

i2

Output units, i

Connection weights, wij

w23

w11

0.1

0.2

w13

w12

1.0

w22

w21

j1

j2

j3

Input units, j

The weights leading into any output node can be represented as a 3-dimensional vector:

w1j = [ 0.1 1.0 0.2 ]

Vectors, Matrices & Neural Nets

i1

i2

Output units, i

Connection weights, wij

w23

-0.9

w11

0.1

0.2

w13

w12

1.0

w22

0.1

w21

1.0

j1

j2

j3

Input units, j

The complete set of weights can be represented as a 3 (row) X 2 (column) matrix:

0.1 1.0 0.21.0 0.1 -0.9

W =

Vectors, Matrices & Neural Nets

i1

i2

Output units, i

Connection weights, wij

Why in the world

would I care??

w23

-0.9

w11

0.1

0.2

w13

w12

1.0

w22

0.1

w21

1.0

j1

j2

j3

Input units, j

The complete set of weights can be represented as a 2 (row) X 3 (column) matrix:

0.1 1.0 0.21.0 0.1 -0.9

W =

Vectors, Matrices & Neural Nets

Because the mathematics of vectors and matrices is well-understood.

Because vectors have a very useful geometric interpretation.

Because Matlab “thinks” in vectors and matrices.

Because you are going to have to learn to think in Matlab.

Why in the world

would I care??

W

Vectors, Matrices & Neural Nets

Because the mathematics of vectors and matrices is well-understood.

Because vectors have a very useful geometric interpretation.

Because Matlab “thinks” in vectors and matrices.

Because you are going to have to learn to think in Matlab.

OK.

Geometric Analysis of Vectors

Dimensionality: The number of numbers in a vector

Geometric Analysis of Vectors

Dimensionality: The number of numbers in a vector

Geometric Analysis of Vectors

Dimensionality: The number of numbers in a vector

Geometric Analysis of Vectors

- Implications for neural networks
- Auto-associative nets
- State of activation at time t is a vector (a point in a space)
- As activations change, vector moves through that space
- Will prove invaluable in understanding Hopfield nets
- Layered nets (“perceptrons”)
- Input vectors activate output vectors
- Points in input space map to points in output space
- Will prove invaluable in understanding perceptrons and back-propagation learning

Multiplying a Vector by a Scalar

8

4

10

5

[ 5 4 ] * 2 = [ 10 8 ]

Lengthens the vector but does not change its orientation

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

If u and v are both row vectors of the same dimensionality…

u = [ 1 2 3 ]

v = [ 4 5 6 ]

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

If u and v are both row vectors of the same dimensionality…

u = [ 1 2 3 ]

v = [ 4 5 6 ]

… then the product

u ·v =

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

If u and v are both row vectors of the same dimensionality…

u = [ 1 2 3 ]

v = [ 4 5 6 ]

… then the product

u ·v = NAN

Is undefined.

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

If u and v are both row vectors of the same dimensionality…

u = [ 1 2 3 ]

v = [ 4 5 6 ]

… then the product

u ·v = NAN

Is undefined.

Huh??

Why??

That’s BS!

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

I told you you’d eventually care about transposing vectors…

?

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

u = [ 1 2 3 ]

v = [ 4 5 6 ]

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

4

5

6

u = [ 1 2 3 ]

v = [ 4 5 6 ]

v’ =

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

4

5

6

u = [ 1 2 3 ]

v’ =

u ·v’ = 32

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

v’

Imagine rotating your row vector into a (pseudo) column vector…

4

5

6

1

2

3

u = [ 1 2 3 ]

u ·v’ = 32

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

v’

Now multiply corresponding elements and add up the products…

4

5

6

1

2

3

4

u = [ 1 2 3 ]

u ·v’ = 32

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

v’

Now multiply corresponding elements and add up the products…

4

5

6

1

2

3

4

10

u = [ 1 2 3 ]

u ·v’ = 32

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

v’

Now multiply corresponding elements and add up the products…

4

5

6

1

2

3

4

10

18

u = [ 1 2 3 ]

u ·v’ = 32

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- The Mantra: “Rows by Columns”
- Multiply rows (or row vectors) by columns (or column vectors)

v’

Now multiply corresponding elements and add up the products…

4

5

6

1

2

3

4

10

1832

u = [ 1 2 3 ]

u ·v’ = 32

Multiplying a Vector by a Vector 1:The Inner Product (aka “Dot Product”)

- Inner product is commutative as long as you transpose correctly

v’

u’

v

4

10

1832

4

5

6

4

5

6

1

2

3

4

10

1832

u = [ 1 2 3 ]

v = [ 4 5 6 ]

u ·v’ = 32

v· u’ = 32

The Inner (“Dot”) Product

- In scalar notation…

- Remind you of…

… the net input to a unit

- In vector notation…

What Does the Dot Product “Mean”?

- Consider uu’

What Does the Dot Product “Mean”?

u’

u

- Consider uu’

u = [ 3, 4 ]

3

4

3

4

9

16

25

5

4

True for vectors of any dimensionality

3

What Does the Dot Product “Mean”?

- What about u v where u v?

What Does the Dot Product “Mean”?

- What about u v where u v?

Well…

… and cos(uv) is a length-invariant measure of the similarity of u and v

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

V’ = [ 1, 1 ]

U = [ 1, 0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

V’ = [ 1, 1 ]

U V = (1 * 1) + (1 * 0) = 1

uv = 45º; cos(uv) = .707

U = [ 1, 0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

V’ = [ 1, 1 ]

U V = (1 * 1) + (1 * 0) = 1

uv = 45º; cos(uv) = .707

U = [ 1, 0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

V’ = [ 1, 1 ]

||u|| = sqrt(1) = 1

||v|| = sqrt(2) = 1.414

uv = 45º; cos(uv) = .707

U = [ 1, 0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

V’ = [ 1, 1 ]

||u|| = sqrt(1) = 1

||v|| = sqrt(2) = 1.414

uv = 45º; cos(uv) = .707

U = [ 1, 0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

V’ = [ 0, 1 ]

uv = 90º; cos(uv) = 0

U = [ 1, 0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

uv = 270º; cos(uv) = 0

U = [ 1, 0 ]

V’ = [ 0, -1 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

uv = 180º; cos(uv) = -1

U = [ 1, 0 ]

V’ = [ -1,0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

uv = 0º; cos(uv) = 1

U = [ 1, 0 ]

V’ = [ 2.2,0 ]

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

In general…

cos(uv) -1…1

True regardless of dimensionality

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

To see why, consider the cosine expressed in scalar notation…

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

… and compare it to the equation for the correlation coefficient…

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

… and compare it to the equation for the correlation coefficient…

if u and v have means of zero, then cos(uv) = r(u,v)

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

… and compare it to the equation for the correlation coefficient…

if u and v have means of zero, then cos(uv) = r(u,v)

The cosine is a special case of the correlation coefficient!

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

… and let’s compare the cosine to the dot product…

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

… and let’s compare the cosine to the dot product…

If u and v have lengths of 1, then the dot product is equal to the cosine.

What Does the Dot Product “Mean”?

- What about u v where u v?

cos(uv) is a length-invariant measure of the similarity of u and v

… and let’s compare the cosine to the dot product…

If u and v have lengths of 1, then the dot product is equal to the cosine.

The dot product is a special case of the cosine, which is a special case of the correlation coefficient, which is a measure of vector similarity!

What Does the Dot Product “Mean”?

- The most common input rule is a dot product between unit i’s vector of weights and the activation vector on the other end
- Such a unit is computing the (biased) similarity between what it expects (wi) and what it’s getting (a).
- It’s activation is a positive function of this similarity

What Does the Dot Product “Mean”?

- The most common input rule is a dot product between unit i’s vector of weights and the activation vector on the other end
- Such a unit is computing the (biased) similarity between what it expects (wi) and what it’s getting (a).
- It’s activation is a positive function of this similarity

ai

asymptotic

ni

What Does the Dot Product “Mean”?

- The most common input rule is a dot product between unit i’s vector of weights and the activation vector on the other end
- Such a unit is computing the (biased) similarity between what it expects (wi) and what it’s getting (a).
- It’s activation is a positive function of this similarity

ai

Step (BTU)

ni

What Does the Dot Product “Mean”?

- It’s activation is a positive function of this similarity

ai

logistic

ni

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

v = [ 4 5 6 ]

u’ =

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

v = [ 4 5 6 ]

u’ =

M =

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

v = [ 4 5 6 ]

u’ =

M =

Row 1

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

v = [ 4 5 6 ]

u’ =

M =

Row 1 times column 1

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

4

v = [ 4 5 6 ]

u’ =

M =

Row 1 times column 1 goes into row 1, column 1

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

4 5

v = [ 4 5 6 ]

u’ =

M =

Row 1 times column 2 goes into row 1, column 2

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

4 5 6

v = [ 4 5 6 ]

u’ =

M =

Row 1 times column 3 goes into row 1, column 3

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

4 56

8

v = [ 4 5 6 ]

u =

M =

Row 2 times column 1 goes into row 2, column 1

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

4 56

8 10

v = [ 4 5 6 ]

u’ =

M =

Row 2 times column 2 goes into row 2, column 2

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

1

2

4 56

8 10 12

v = [ 4 5 6 ]

u’ =

M =

Row 2 times column 3 goes into row 2, column 3

Multiplying a Vector by a Vector 2:The Outer Product

The two vectors need not have the same dimensionality.

Same Mantra: Rows by Columns.

This time, multiply a column vector by a row vector:

M = u’ * v

A better way to visualize it…

v = [ 4 5 6 ]

4 56

8 10 12

1

2

u’ =

= M

Multiplying a Vector by a Vector 2:The Outer Product

Outer product is not exactly commutative…

M = u’ * v

M = v’ * u

u = [ 1 2 ]

v = [ 4 5 6 ]

4

5

6

4 5 6

8 10 12

1

2

4 8

5 10

6 12

u’ =

= M

v’ =

Multiplying a Vector by a Matrix

- Same Mantra: Rows by Columns

Rows by Columns

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

Rows by Columns

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

Multiply rows

Rows by Columns

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

Multiply rows by columns

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

Make a proxy column vector…

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

.2

.6

.3

.7

.9

.4

.3

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

.2

.6

.3

.7

.9

.4

.3

Now compute the dot product of the (proxy) row vector with each column of the matrix…

[ 1.5]

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

.2

.6

.3

.7

.9

.4

.3

[ 1.5 1.4]

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

.2

.6

.3

.7

.9

.4

.3

[ 1.5 1.4 0.8]

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

.2

.6

.3

.7

.9

.4

.3

[ 1.5 1.4 0.8 1.5]

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

.2

.6

.3

.7

.9

.4

.3

[ 1.5 1.4 0.8 1.5 1.9]

Each such multiplication is a simple dot product

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

.2

.6

.3

.7

.9

.4

.3

[ 1.5 1.4 0.8 1.5 1.9 1.2]

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

The result is a row vector with as many columns (dimensions) as the matrix (not the vector)

[ 1.5 1.4 0.8 1.5 1.9 1.2]

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

7-dimensional vector

[ 1.5 1.4 0.8 1.5 1.9 1.2]

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

7-dimensional vector

6-dimensional vector

[ 1.5 1.4 0.8 1.5 1.9 1.2]

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

7-dimensional vector

7 (rows) X 6 (columns) matrix

6-dimensional vector

[ 1.5 1.4 0.8 1.5 1.9 1.2]

A row vector:

[ .2 .6 .3 .7 .9 .4 .3 ]

A matrix:

.3 .4 .8 .1 .2 .3

.5 .2 0 .1 .5 .2

.1 .1 .9 .2 .5 .3

.2 .4 .1 .7 .8 .5

.9 .9 .2 .5 .3 .5

.4 .1 .2 .7 .8 .2

.1 .2 .2 .5 .7 .2

7-dimensional vector

7 (rows) X 6 (columns) matrix

6-dimensional vector

[ 1.5 1.4 0.8 1.5 1.9 1.2]

Multiplying a Matrix by a Matrix

- The Same Mantra: Rows by Columns

Row 1 X Column 1

A 2 X 3 matrix

A 3 X 2 matrix

1

2

3

1 2

1 2

1 2

Column 1

Row 1

1 2 3

1 2 3

Result = 6

Place the result in row 1,

Row 1 X Column 1

A 2 X 3 matrix

A 3 X 2 matrix

1

2

3

1 2

1 2

1 2

Column 1

Row 1

1 2 3

1 2 3

Result = 6

Place the result in row 1, column 1

Row 1 X Column 1

A 2 X 3 matrix

A 3 X 2 matrix

1

2

3

1 2

1 2

1 2

Column 1

Row 1

1 2 3

1 2 3

Result = 6

Place the result in row 1, column 1 of a new matrix…

6

Row 1 X Column 2

A 2 X 3 matrix

A 3 X 2 matrix

1

2

3

1 2

1 2

1 2

Column 2

Row 1

1 2 3

1 2 3

Result = 12

Place the result in row 1, column 2 of the new matrix…

6 12

Row 2 X Column 1

A 2 X 3 matrix

A 3 X 2 matrix

1

2

3

1 2

1 2

1 2

Column 1

1 2 3

1 2 3

Row 2

Result = 6

Place the result in row 2, column 1 of the new matrix…

6 12

6

Row 2 X Column 2

A 2 X 3 matrix

A 3 X 2 matrix

1

2

3

1 2

1 2

1 2

Column 2

1 2 3

1 2 3

Row 2

Result = 12

Place the result in row 2, column 2 of the new matrix…

6 12

6 12

So…

A 2 X 3 matrix

A 3 X 2 matrix

1 2

1 2

1 2

1 2 3

1 2 3

*

The result has the same number of rows as the first matrix…

=

6 12

6 12

A 2 X 2 matrix

So…

A 2 X 3 matrix

A 3 X 2 matrix

1 2

1 2

1 2

1 2 3

1 2 3

*

The result has the same number of rows as the first matrix…

…and the same number of columns as the second.

=

6 12

6 12

A 2 X 2 matrix

…and…

A 2 X 3 matrix

A 3 X 2 matrix

1 2

1 2

1 2

1 2 3

1 2 3

*

…and the number of columns in the first matrix…

=

6 12

6 12

A 2 X 2 matrix

…and…

A 2 X 3 matrix

A 3 X 2 matrix

1 2

1 2

1 2

1 2 3

1 2 3

*

…and the number of columns in the first matrix…

…must be equal to the number of rows in the second.

=

6 12

6 12

A 2 X 2 matrix

This is basic (default) matrix multiplication.

There’s other more complicated stuff, too.

You (probably) won’t need it for this class.

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