# Linear Algebra Notes - Definitions & Theorems

Information is based on Linear Algebra with Applications by Jeffrey Holt.

## System of Equations

### Definition 1.1

A system of linear equations is a collection of equations of the form

### Theorem 1.2

A system of linear equations has no solutions, exactly one solution, or infinitely many solutions.

### Definition 1.3

A linear system is in echelon form if

1. Every variable is the leading variable of at most one equation.
2. The system is organized in a descending “stair step” pattern so that the index of the leading variables increases from the top to bottom.
3. Every equation has a leading variable.

### Definition 1.4

A matrix is in echelon form if

1. Every leading term is in a column the left of the leading term of the row below it
2. Any zero rows are at the bottom of the matrix

### Definition 1.5

A matrix is in reduced (row) echelon form (RREF) if

1. it is in echelon form
2. all pivot positions contain a “1”
3. the only nonzero term in a pivot column is in the pivot position

### Theorem 1.6

A given matrix is equivalent to a unique matrix that is in reduced row echelon form.

## Vectors

### Definition 2.1

A vector is an ordered list of real numbers $u_1, u_2, …, u_n$ expressed as

or as $\pmb{u}=(u_1, u_2, …, u_n)$. The set of all vectors with n entries is denoted by $\mathbb{R}^n$.

### Definition 2.2

Let $\pmb{u}$ and $\pmb{v}$ be vectors in $\mathbb{R}^n$ given by

Suppose that $c$ is a real number, called a scalar in this context. Then we have the following definitions:

Equality $\pmb{u} = \pmb{v}$ if and only if $u_1 = v_1$, $u_2 = v_2$…

Addition $\pmb{u} + \pmb{v} = \begin{bmatrix} u_1 \\ u_2 \\ ... \\ u_n \end{bmatrix} + \begin{bmatrix} v_1 \\ v_2 \\ ... \\ v_n \end{bmatrix} = \begin{bmatrix} u_1 + v_1 \\ u_2 + v_2 \\ ... \\ u_n + v_n \end{bmatrix}$

Scalar Multiplication $c\pmb{u} = c\begin{bmatrix} u_1 \\ u_2 \\ ... \\ u_n \end{bmatrix} = \begin{bmatrix} c\cdot u_1 \\ c\cdot u_2 \\ ... \\ c\cdot u_n \end{bmatrix}$

The set of all vectors in $\mathbb{R}^n$, taken together with these definitions of addition and scalar multiplication, is called Euclidean space.

### Theorem 2.3: Algebraic Properties of Vectors

Let $a$ and $b$ be scalars, and $\pmb{u}$, $v$, and $w$ be vectors in $\mathbb{R}^n$. Then

1. $\pmb{u} + \pmb{v} = \pmb{v} + \pmb{u}$
2. $a(\pmb{u} + \pmb{v}) = a\pmb{u} + a\pmb{v}$
3. $(a + b) \pmb{u} = a\pmb{u} + b\pmb{u}$
4. $(\pmb{u} + \pmb{v}) + \pmb{w} = \pmb{u} + (\pmb{v} + w)$
5. $a(b\pmb{u}) = (ab)\pmb{u}$
6. $\pmb{u} + (-\pmb{u}) = 0$
7. $\pmb{u} + 0 = 0 + \pmb{u} = \pmb{u}$
8. $l\pmb{u} = \pmb{u}$

### Definition 2.4

If $u_1, u_2, …, u_m$ are vectors and $c_1, c_2, …, c_m$ are scalars, then

is a linear combination of the vectors. Note that it is possible for scalars to be negative or equal to zero,

### Definition 2.5

Let ${u_1, u_2, …, u_m}$ be a set of vectors in $\mathbb{R}^n$. The span of this set is denoted $span{u_1, u_2, …, u_m}$and its defined to be the set of all linear combinations

where $x_1, x_2, …, x_m$ can be any real numbers.

### Theorem 2.6

Let $\pmb{u}_1, \pmb{u}_2, …, \pmb{u}_m$ and $\pmb{v}$ be vectors in $\mathbb{R}^n$. Then $\pmb{v}$ is an element of $span{\pmb{u_1}, \pmb{u_2}, …, \pmb{u_m}}$ if and only if the linear system represented by the augmented matrix

has a solution.

### Theorem 2.7

Let $\pmb{u}_1, \pmb{u}_2, …, \pmb{u}_m$ and $\pmb{u}$ be vectors in $\mathbb{R}^n$. If $\pmb{u}$ is in $span{\pmb{u}_1, \pmb{u}_2, …, \pmb{u}_m}$, then $span{\pmb{u}, \pmb{u}_1, \pmb{u}_2, …, \pmb{u}_m} = span{\pmb{u}_1, \pmb{u}_2, …, \pmb{u}_m}$.

### Theorem 2.8

Let ${\pmb{u}_1, \pmb{u}_2, …, \pmb{u}_m}$ be a set of vectors in $\mathbb{R}^n$. If $m < n$, then this set does not span $\mathbb{R}^n$. If $m \geq n$, then the set might span $\mathbb{R}^n$ or it might not. In this case, we cannot say more without additional information about the vectors.

### Definition 2.9

Let $\pmb{a}_1, \pmb{a}_2, …, \pmb{a}_m$ be vectors in $\mathbb{R}^n$. If

then $A \pmb{x} = x_1 \pmb{a}_1 + x_2 \pmb{a}_2 + … + x_m \pmb{a}_m$.

### Theorem 2.10

Let $\pmb{a}_1, \pmb{a}_2,…, \pmb{a}_m$ and $\pmb{b}$ be vectors in $\mathbb{R}^n$. Then the following statements are equivalent. That is, if one is true, then so are the others, and if one is false, then so are the others.

1. $\pmb{b}$ is in $span{\pmb{a}1,\pmb{a}2,…,\pmb{a}m}$.
2. The vector equation $x_1 \pmb{a}_1 + x_2 \pmb{a}_2 + … + x_m \pmb{a}_m = \pmb{b}$ has at least one solution.
3. The linear system corresponding to $\begin{bmatrix} \pmb{a}_1 & \pmb{a}_2 & … & \pmb{a}_m & \pmb{b}\end{bmatrix}$ has at least one solution.
4. The equation $A \pmb{x} = \pmb{b}$, with $A$, and $\pmb{x}$ given as in Definition 2.9.

### Definition 2.11

Let ${ \pmb{u}_1, \pmb{u}_2, …, \pmb{u}_m}$ be a set of vectors in $\mathbb{R}^n$. If the only solution to the vector equation

is the trivil solution given by $x_1 = x_2 = … = x_m = 0$, then the set ${ \pmb{u}_1 \pmb{u}_2 … \pmb{u}_m}$ is linearly independent. If there are nontrivial solutions, then the set is linearly dependent.

### Theorem 2.12

Suppose that ${\pmb{-}, \pmb{u}_1 , \pmb{u}_2 , … , \pmb{u}_m}$ is a set of vectors in $\mathbb{R}^n$. Then the set is linearly dependent.

### Theorem 2.14

Let ${\pmb{u}_1 , \pmb{u}_2 , … , \pmb{u}_m}$ be a set of vectors in $\mathbb{R}^n$. Then the set is linearly dependent if and only if one of the vectors in the set is in the span of the other vectors.

### Theorem 2.15

Let $A = \begin{bmatrix} \pmb{a}_1 & \pmb{a}_2 & … & \pmb{a}_m \end{bmatrix}$ and $\pmb{x} = (x_1 , x_2 , … , x_m)$. The set ${ \pmb{a}_1 , \pmb{a}_2 , … , \pmb{a}_m}$ is linearly independent if and only if the homogeneous linear system

has only the trivial solution.

### Theorem 2.16

Suppose that $A = \begin{bmatrix} \pmb{a}_1 & \pmb{a}_2 & … & \pmb{a}_m \end{bmatrix}$, and let $\pmb{x} = (x_1 , x_2 , … , x_m)$ and $\pmb{y} = y_1 , y_2 , … , y_m$. Then

1. $A (\pmb{x} + \pmb{y} ) = A \pmb{x} + A \pmb{y}$
2. $A (\pmb{x} - \pmb{y} ) = A \pmb{x} - A \pmb{y}$

### Theorem 2.17

Let $\pmb{x}_p$ be a particular solution to

Then all solution $\pmb{x}_g$ to it have the form $\pmb{x}_g - \pmb{x}_p + \pmb{x}_h$, where $\pmb{x}_h$ is a solution to the associated homogeneous system $A \pmb{x} = \pmb{0}$.

### Theorem 2.19: The Big Theorem - Version 1

Let $\mathcal{A} = { \pmb{a}_1 , \pmb{a}_2 , … , \pmb{a}_m}$ be a set of $n$ vectors in $\mathbb{R}^n$, an dlet $A = \begin{bmatrix} \pmb{a}_1 & \pmb{a}_2 & … & \pmb{a}_m \end{bmatrix}$. Then the followign are equivalent:

1. $\mathcal{A}$ spans $\mathbb{R}^n$.
2. $\mathcal{A}$ is linearly independent.
3. $A \pmb{x} = \pmb{b}$ has a unique solution for all $\pmb{b}$ in $\mathbb{R}^n$.

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