# 09-240/Classnotes for Thursday September 10

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## Date: Thurs. Sept. 10, 2009

• Professor's name: Dror Bar-Natan
• Solve systems of equations

5x1 − 2x2 + x3 = 9

x1 + x2x3 = 2

2x1 + 9x2 − 3x3 = − 4

• how? when? one/many?
• This describes the small-scale behaviour of almost everything that has a mathematical description.

1. A matrix is a square or rectangular array of numbers.

$\begin{pmatrix} 5 & -2 & 1\\ -1 & 1 & -1\\ 2 & 9 & -3 \end{pmatrix}$
• we will learn addition, multiplication, and powers of matrices
$\mathbf{A}=\begin{pmatrix} 5 & -2 & 1\\ -1 & 1 & -1\\ 2 & 9 & -3 \end{pmatrix}, \mathbf{B}=\cdots$
$\begin{pmatrix} 5 & -2 & 1\\ -1 & 1 & -1\\ 2 & 9 & -3 \end{pmatrix}+\mathbf{B}$

$\mathbf{AB} \neq \mathbf{BA}$

$\mathbf{A}^{2009}$

• describes the approximate long-term behaviour of almost anything...
• Do all this without choosing coordinates.

2. Do everything over other “systems of numbers”

1. real numbers
2. rational numbers
3. complex numbers (things like alternating current, circuit)
4. {0,1} (binary, computer science)

3. Hidden Agenda

• Learn the basic pure-math processes of: abstraction, generalizations, definitions, theorems, proofs, notation logic

• can add things to wiki (so long as relevant to course material)
• HW assigned on Tuesday, due in tutorial 9 days later.
• HW graded and returned by following tutorial

5. Classwork done today

• The Real Numbers: a set $\mathbb{R}$ with two binary operations $\,\!+$, $\times$(2 inputs, one output) and also with two distinguished elements $0,1\epsilon\mathbb{R}$ with the following properties:

R1 $\forall a,b$

1. $\,\!a + b = b + a$
2. $a \cdot b=b \cdot a$

Aside: The $\perp$ character used for additon:

• Prof. Dror asked why + is sometimes written as $\perp$?