The fundamental problem of Linear Algebra is solving a system of Linear Equations.

- A system of linear equations is a system of first order equations with multiple unknowns.

Suppose we have a system with $m$ equations and $n$ unknowns:

$\left\{\begin{matrix} a_{11} x_1 + a_{12} x_2 + \ ... \ + a_{1n} x_n = b_1\\ a_{21} x_1 + a_{22} x_2 + \ ... \ + a_{2n} x_n = b_2\\ \vdots \\ a_{m1} x_1 + a_{22} x_2 + \ ... \ + a_{mn} x_n = b_m\\ \end{matrix}\right.$

The coefficients of the unknowns form a Matrix - a rectangular array of numbers:

- [math]A = \begin{bmatrix} a_{11} & a_{12} & ... & a_{1n}\\ a_{21} & a_{22} & ... & a_{2n}\\ \vdots & \vdots & \vdots & \vdots \\ a_{m1} & a_{m2} & ... & a_{mn} \end{bmatrix}[/math]
- We call this matrix $A$, it's $m \times n$ matrix: with $m$ rows and $n$ columns

The unknowns themselves form a column vector $\mathbf{x}$, and the ... form a column vector $b$

- [math]\mathbf{x} = \begin{bmatrix} x_1 \\ x_2 \\ \vdots \\ x_n \\ \end{bmatrix}[/math], [math]\mathbf{b} = \begin{bmatrix} b_1 \\ b_2 \\ \vdots \\ b_m \\ \end{bmatrix}[/math],
- they're both column vectors, $\mathbf{x}$ has size $n$, and $\mathbf{b}$ has size $m$
- can write them as rows using the transposition notation: $\mathbf{x} = [x_1, x_2, ..., x_n]^T$ (but they still remain column vectors)

So the system of linear equations can be expressed in a matrix form as $A\mathbf{x} = \mathbf{b}$

Suppose we have the following system of linear equations:

$\left\{\begin{matrix} 2x - y = 0\\ -x + 2y = 3 \end{matrix}\right.$

Matrix form:

- [math]\begin{bmatrix} 2 & -1 \\ -1 & 2 \end{bmatrix} \begin{bmatrix} x \\ y \end{bmatrix} = \begin{bmatrix} 0 \\ 3 \end{bmatrix}[/math]

We can see this in two possible ways:

- as a crossing of lines (hyperplanes) - the row picture
- as vectors - the column picture

We have two lines:

- $2x - y = 0$, $-x + 2y = 3$
- plot them, and the point where they cross is the solution $\mathbf{x}$

The solution is $[x, y]^T = [1, 2]^T$

For 3 and more dimensions, we have (hyper)planes instead of lines.

- But it's the same: we want to find a point where they cross
- File:Secretsharing 3-point.svg

We have two vectors:

- [math]x \begin{bmatrix} 2 \\ -1 \end{bmatrix} + y \begin{bmatrix} -1 \\ 2 \end{bmatrix} = \begin{bmatrix} 0 \\ 3 \end{bmatrix}[/math]

We need to combine first two vectors [math]v = \begin{bmatrix} 2 \\ -1 \end{bmatrix}[/math] and [math]u = \begin{bmatrix} -1 \\ 2 \end{bmatrix}[/math] so they form [math]\begin{bmatrix} 0 \\ 3 \end{bmatrix}[/math]

I.e. we want to find a Linear Combination of these columns.

- From the vector picture we know that the solution is $[1, 2]^T$, so let's take
- so we take 1 of vector $\mathbf u$ and 2 of vector $\mathbf v$ and end up at exactly $\mathbf b$!

Python code to reproduce the figures

import matplotlib.pylab as plt import numpy as np class Line: def __init__(self, slope, intercept): self.slope = slope self.intercept = intercept def calculate(self, x1): x2 = x1 * self.slope + self.intercept return x2 line1 = Line(2, 0) line2 = Line(0.5, 1.5) a = np.array([-6, 6]) plt.plot(a, line1.calculate(a), color='red', marker='') plt.plot(a, line2.calculate(a), color='blue', marker='') plt.scatter([1], [2]) plt.grid() plt.axis('equal') plt.ylim([-1, 3]) plt.xlim([-1, 3])

import matplotlib.pylab as plt plt.axis('equal') plt.quiver([0, 0, 0], [0, 0, 0], [-1, 2, 0], [2, -1, 3], color=['red', 'blue', 'black'], angles='xy', scale_units='xy', scale=1) plt.quiver([2], [-1], [-2], [4], color='red', width=0.005, scale=1, angles='xy', scale_units='xy') plt.grid() plt.ylim([-2, 5]) plt.xlim([-4, 4]) plt.show()

How we can solve the system $A \mathbf x = \mathbf b$?

- The easiest way: Gaussian Elimination - elimination of variables
- this is the row picture

Vector Solution: the solution found with the column picture

Matrix Solution

- if $A$ is non-singular and invertible, then to find $\mathbf x$ multiply $\mathbf b$ by the inverse of $A$: $\mathbf x = A^{-1} \mathbf b$
- if $\mathbf b \in C(A)$ - Column Space of $A$
- this is especially important when the number of columns $n$ is less than the number of rows $m$

Such systems are called *homogeneous* - see Homogeneous Systems of Linear Equations

Let $A$ be $n \times m$ matrix of rank $r$

- we know that the system has a solution if $\mathbf b \in C(A)$

Steps:

- reduce $A$ to Row Reduced Echelon Form
- set all free variables to 0 and solve - get $\textbf x_p = \textbf x_\text{particular}$
- then solve $A \mathbf x_n = \mathbf 0$ - get all $\mathbf x_n$ - all $\mathbf x$ that solve the homogeneous system
- Then find all other solutions: they are $\mathbf x = \textbf x_p + \mathbf x_n$
- this solution is called
*the complete solution* - why? $A \mathbf x_p = \mathbf b$ and $A \mathbf x_n = \mathbf 0$. Add them and get $A \cdot (\mathbf x_p + \mathbf x_n) = \mathbf b + \mathbf 0 = \mathbf b$
- so we can the solution as the Nullspace $C(A)$ but shifted away from the origin by $x_p$
- note that this solution doesn't form a subspace

Let $A$ be $m \times n$ matrix of rank $r$

- $m$ - rows, $n$ - cols
- $r \leqslant m$, $r \leqslant n$

Full rank = $r$ is as big as it can be

- suppose that $n \leqslant m$, i.e. the number of columns is smaller than the number of rows
- so full column rank matrix = $r = n$
- there's a pivot in every column, $n$ pivots $\Rightarrow$ no free variables

Nullspace $N(A)$?

- there are no free variables to consider
- so $N(A) = \{ \ \mathbf 0 \ \}$

Solution to $A \mathbf x = \mathbf b$:

- if the solution exists (i.e. $\mathbf b \in C(A)$) then this solution is unique
- so we have either 0 solutions or 1

Solving $A\mathbf x = \mathbf b$

- As metioned earlier - need to reduce it to RREF
- Can do that with Gaussian Elimination via LU Decomposition
- Let $LU = A$
- then $A \mathbf x = \mathbf b$ becomes $LU \mathbf x = \mathbf b$
- Now let $U \mathbf x = \mathbf y$ and $L \mathbf y = \mathbf x$
- With this substitution we need to solve two equations now, but they are simple because $L$ is lower-diagonal and $U$ is upper-diagonal

No solution?

- Can approximate it with Normal Equation
- That would be the Least Squares solution

- Now suppose that $m \leqslant n$ and $r = m$
- so every row has a pivot, but only $r$ columns have pivots, the remaining $n - r$ don't
- so there are $r$ pivot columns, and $n - r$ free columns

for which $\mathbf b$ we can solve $A \mathbf x = \mathbf b$?

- there are no zero rows, so can solve the system for any $\mathbf b$

- If our $A$ is square, $m = n = r$
- then there's always a solution, and $A$ is called
*invertible* - $N(A) = \{ \ \mathbf 0 \ \}$ - there's only one unique solution

- $A \mathbf x = \mathbf 0$ always have a solution - there's always something in the Nullspace $N(A)$ of $A$ apart from the zero-vector
- reason: there are always free variables and we can assign any non-zero values to them and solve the homogeneous system

Zero or $\infty$ solutions

- Linear Algebra MIT 18.06 (OCW)
- Курош А.Г. Курс Высшей Алгебры
- http://en.wikipedia.org/wiki/System_of_linear_equations