# 數學 - linear algebra review

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## Matrices and Vectors : article

Matrix : Rectangular array of numbers

``````# Dimension of matrix: number of x number of columns

|2 3|
|1 3|  => 3 x 2 matrix
|0 1|
``````

Martix Elements(entries of matrix)

``````# Aij = " 'i,j entry' in the ith row, jth column. "

|12 14  -3  6|
A =| 2 18 -10 12|
| 1 -9 -10 -2|
| 3  3  6   1|

A2,4 = 12
A1,3 = -3
A4,2 = 3
A5,1 = undefined (error)
``````

Vector: An n x 1 matrix

``````# 4 dimensional vector

|460|
y = |232|
|315|
|178|
-------------------------------
# yi = ith element

y1 = 460
y2 = 232
-------------------------------
# 1-indexed vs 0-indexed:

|y1|      |y0|
y = |y2|  y = |y1|
|y3|      |y2|
|y4|      |y3|
1-indexed   0-indexed
``````

## Addition and Scalar Multiplication : article

Matrix Addition

``````|1 0|   |4 0.5|   |5 0.5|
|2 5| + |2   5| = |4  10|
|3 1|   |0   1|   |3   2|
``````

Scalar Multiplication

Scalar = real number

``````    |1 0|   |3  0|   |1 0|
3 x |2 5| = |6 15| = |2 5| x 3
|3 1|   |9  3|   |3 1|

|4 0|            |4 0|   |  1   0 |
|6 3| / 4  = 1/4 |6 3| = | 3/2 3/4|
``````

Combination of Operands

``````# x: Scalar multiplication
# /: Scalar division

|1|   |0|   |3|
3 x |4| + |0| - |0| / 3
|2|   |5|   |2|

# +: matrix addition / vector addition
# -: matrix subtraction / vector subtraction

| 3|   |0|   | 1 |
= |12| + |0| - | 0 |
| 6|   |5|   |2/3|

|  2 |
= | 12 | --> # 3 x 1 matrix / 3-dimensional vector
|31/3|
``````

## Matrix Vector Multiplication : article

``````# Example

|1 3|               | 16 |
|4 0|       |1|  =  |  4 |
|2 1|   x   |5|     |  7 |
3 x 2     2 x 1    3 x 1  matrix

# Details:
A             X         x　          =            y
| 　      |                |   |   　       　       |   |
| 　      |        X       |   |   　     =　        |   |
| 　      |                |   |   　       　       |   |
m x n matrix              n x 1 matrix
(m rows, n cloumns)       (n-dimentional vector)    m-dimensional vector
``````

Hypothesis Example:

``````House sizes:
2104
1416               hθ(x) = -40 + 0.25x
1534
852

(4,2) Matrix      (2,1) vector           (4,1) Matrix

| 1  2104 |                        | -40 x 1 + 2104 x 0.25 |
| 1  1416 |   x   | -40  |    =    | -40 x 1 + 1416 x 0.25 |
| 1  1534 |       | 0.25 |         | -40 x 1 + 1534 x 0.25 |
| 1   852 |                        | -40 x 1 +  852 x 0.25 |

DataMatrix    x    Parameters   =   prediction

``````

## Matrix Matrix Multiplication : article

``````     A             X           B      　        =              C
| 　      |                |         |   　       　       |       |
| 　      |        X       |         |   　     =　        |       |
| 　      |                |         |   　       　       |       |
m x n matrix              n x o matrix
(m rows, n cloumns)       (n row, o columns)               m x o  matrix

``````

Example:

``````|1  3|   |0 1|    |  9   7 |
|2  5| x |3 2|  = | 15  12 |

# 可拆成這樣看：

|1  3|   |0|    | 1*0 + 3*3 |    | 9|
|2  5| x |3|  = | 2*0 + 5*3 | =  |15|

|1  3|   |1|    | 1*0 + 3*3 |     | 7|
|2  5| x |2|  = | 2*0 + 5*3 |  =  |12|
``````

Hypothesis Example:

``````House sizes:         Have 3 competing hypotheses:
2104
1416               1. hθ(x) = -40 + 0.25x
1534               2. hθ(x) = 200 + 0.1x
852                3. hθ(x) = -150 + 0.4x

Matrix          Matrix                        Matrix

| 1  2104 |                                | 486 410 692 |
| 1  1416 |   x   | -40  200 -150 |   =    | 314 342 416 |
| 1  1534 |       | 0.25 0.1  0.4 |        | 344 353 464 |
| 1   852 |                                | 173 285 191 |

DataMatrix    x    Parameters   =       prediction
| (prediction of 1:hθ) (prediction of 2:hθ) (prediction of 3:hθ)|
| (prediction of 1:hθ) (prediction of 2:hθ) (prediction of 3:hθ)|
| (prediction of 1:hθ) (prediction of 2:hθ) (prediction of 3:hθ)|
| (prediction of 1:hθ) (prediction of 2:hθ) (prediction of 3:hθ)|
``````

## Matrix Multiplication Properties : article

Not Coummutative:

note: A and B be matrices. Then in general, A x B != B x A (not commutative)

``````# Example:

A    x   B

|1  1|   |0 0|    | 2  0 |
|0  0| x |2 0|  = | 0  0 |

|0  0|   |1  1|    | 0  0 |
|2  0| x |0  0|  = | 2  2 |

# A and B are matrices
A x B  !=  B x A

``````

Associative:

``````3 x 5 x 2

# Associative:
3 x ( 5 x 2 ) = ( 3 x 5 ) x 2
3 x 10 = 30 = 15 x 2

Matirx:
A x B x C
Let D = B x c, Compute A x D
Let E = A x B, Compute E x C
``````

Identity Matrix

1 is identiy, 1 x Z = Z x 1 = Z ( for any z )

``````# Denoted I (or I nxn)
# Examples of identity matrices:

2 x 2        3 x 3         4 x 4
| 1 0 |     | 1 0 0 |     | 1 0 0 0 |
| 0 1 |     | 0 1 0 |     | 0 1 0 0 |
| 0 0 1 |     | 0 0 1 0 |
| 0 0 0 1 |

For any matrix A:
A     *   I    =   I   *    A    =   A
(m, n)  (n, n)   (m, m)   (m, m)   (m, n )
``````

## Inverse and Transpose : article

Inverse:

``````1: identity
3 x ( 3^-1 ) = 1
12 x ( 12^-1 ) = 1
``````

Not all numbers have an inverse.

``````0(0^-1) ==> (0^0-1): undefined
``````

Matrix inverse:

if A is an `m x m [square matrix]` matrix, and if it has an inverse,

``````if A is an m x m matrix, and if it has an inverse,

A(A^-1) = (A^-1)A = I
``````

m x m `[square matrix]`

Example:

``````|3  4|   |  0.4   -0.1 |    | 1  0 |
|2 16| x |-0.005  0.075|  = | 0  1 | =  I 2x2
A             A^-1          A^-1A
``````
• Matrices that don’t have an inverse are “singular” or “degenerate”

Matrix Transpose:

``````Example:
A  =  |1 2 0|      A^T = |1  3|
|3 5 9|            |2  5|
|0  9|

Let A be an m x n matrix, and let B = A^T.
Then B is an n x m matrix, and

B(i,j) = A(j,i)

``````

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