# 09-240/Classnotes for Tuesday September 15

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(In the above gallery, there is a complete set of notes for the lecture given by Professor Natan on September 15th in PDF form.)

The real numbers A set $\mathbb R$ with two binary operators and two special elements $0, 1 \in \mathbb R$ s.t.

$F1.\quad \forall a, b \in \mathbb R, a + b = b + a \mbox{ and } a \cdot b = b \cdot a$
$F2.\quad \forall a, b, c, (a + b) + c = a + (b + c) \mbox{ and } (a \cdot b) \cdot c = a \cdot (b \cdot c)$
$\mbox{(So for any real numbers } a_1, a_2, ..., a_n, \mbox{ one can sum them in any order and achieve the same result.}$
$F3.\quad \forall a, a + 0 = a \mbox{ and } a \cdot 1 = a$
$F4.\quad \forall a, \exists b, a + b = 0 \mbox{ and } \forall a \ne 0, \exists b, a \cdot b = 1$
$\mbox{So } a + (-a) = 0 \mbox{ and } a \cdot a^{-1} = 1$
$\mbox{(So } (a + b) \cdot (a - b) = a^2 - b^2)$
$\forall a, \exists x, x \cdot x = a \mbox{ or } a + x \cdot x = 0$
Note: or means inclusive or in math.
$F5.\quad (a + b) \cdot c = a \cdot c + b \cdot c$

Definition: A field is a set F with two binary operators $\,\!+$: F×FF, $\times\,\!$: F×FF and two elements $0, 1 \in \mathbb R$ s.t.

$F1\quad \mbox{Commutativity } a + b = b + a \mbox{ and } a \cdot b = b \cdot a \forall a, b \in F$
$F2\quad \mbox{Associativity } (a + b) + c = a + (b + c) \mbox{ and } (a \cdot b) \cdot c = a \cdot (b \cdot c)$
$F3\quad a + 0 = a, a \cdot 1 = a$
$F4\quad \forall a, \exists b, a + b = 0 \mbox{ and } \forall a \ne 0, \exists b, a \cdot b = 1$
$F5\quad \mbox{Distributivity } (a + b) \cdot c = a \cdot c + b \cdot c$

## Examples

1. $F = \mathbb R$
2. $F = \mathbb Q$
3. $\mathbb C = \{ a + bi : a, b \in \mathbb R \}$
$i = \sqrt{-1}$
$\,\!(a + bi) + (c + di) = (a + c) + (b + d)i$
$\,\!0 = 0 + 0i, 1 = 1 + 0i$
4. $\,\!F_2 = \{ 0, 1 \}$
5. $\,\!F_7 = \{ 0, 1,2,3,4,5,6 \}$
6. $\,\!F_6 = \{ 0, 1,2,3,4,5 \}$ is not a field because not every element has a multiplicative inverse.
Let $a = 2.$
Then $a \cdot 0 = 0, a \cdot 1 = 2, a \cdot 3 = 0, a \cdot 4 = 2, a \cdot 5 = 4$
Therefore F4 fails; there is no number b in F6 s.t. a · b = 1
Ex. 4
+ 0 1
0 0 1
1 1 0
Ex. 4
× 0 1
0 0 0
1 0 1
Ex. 5
+ 0 1 2 3 4 5 6
0 0 1 2 3 4 5 6
1 1 2 3 4 5 6 0
2 2 3 4 5 6 0 1
3 3 4 5 6 0 1 2
4 4 5 6 0 1 2 3
5 5 6 0 1 2 3 4
6 6 0 1 2 3 4 5
Ex. 5
× 0 1 2 3 4 5 6
0 0 0 0 0 0 0 0
1 0 1 2 3 4 5 6
2 0 2 4 6 1 3 5
3 0 3 6 2 5 1 4
4 0 4 1 5 2 6 3
5 0 5 3 1 6 4 2
6 0 6 5 4 3 2 1

Theorem: F2 is a field.

In order to prove that the associative property holds, make a table (similar to a truth table) for a, b and c.

a b c
0 0 0
0 0 1
0 1 0
0 1 1 (0 + 1) + 1 =? 0 + (1 + 1)
1 + 1 =? 0 + 0
0 = 0
1 0 0
1 0 1
1 1 0
1 1 1

Theorem: $\,\! F_p$ for $p > 1$ is a field iff (if and only if) $p$ is a prime number

Proof:

$a \in \mathbb Z$ has a multiplicative inverse modulo $m$ if and only if a and m are relatively prime.

This can be shown using Bézout's identity:

$\exists x, y \mbox{ s.t. } ax + my = 1$
$\left(ax + my\right) \pmod{m} = 1\pmod{m}$
$ax = 1$
$x = a^{-1}$

We have shown that $a$ has a multiplicative inverse modulo m if $a$ and $m$ are relatively prime. It is therefore a natural conclusion that if $m$ is a prime number all elements in the set will be relatively prime to m.

$23 \times 27 = \begin{matrix} 27 \\ \overbrace{23 + 23 + 23 + \cdots + 23} \end{matrix} = 621$

$27 \times 23 = \begin{matrix} 23 \\ \overbrace{27 + 27 + 27 + \cdots + 27} \end{matrix} = 621$

One may interpret this as counting the units in a 23×27 rectangle; one may choose to count along either 23 rows or 27 columns, but both ways lead to the same answer.

Exponentiation is repeated multiplication, but it does not have the same properties as multiplication; 23 = 8, but 32 = 9.

## Tedious Theorem

1. $a + b = c + b \Rightarrow a = c$ "cancellation property"
Proof:
By F4, $\exists d \mbox{ s.t. } b + d = 0$
$\,\! (a + b) + d = (c + b) + d$
$\Rightarrow a + (b + d) = c + (b + d)$ by F2
$\Rightarrow a + 0 = c + 0$ by choice of d
$\Rightarrow a = c$ by F3
2. $a \cdot b = c \cdot b , (b \ne 0) \Rightarrow a = c$
3. $a + O' = a \Rightarrow O' = 0$
Proof:
$\,\! a + O' = a$
$\Rightarrow a + O' = a + 0$ by F3
$\Rightarrow O' = 0$ by adding the additive inverse of a to both sides
4. $a \cdot l' = a, a \ne 0 \Rightarrow l' = 1$
5. $a + b = 0 = a + b' \Rightarrow b = b'$
6. $a \cdot b = 1 = a \cdot b' \Rightarrow b = b' = a^{-1}$
$\,\! \mbox{Aside: } a - b = a + (-b)$
$\frac ab = a \cdot b^{-1}$
7. $\,\! -(-a) = a = (a^{-1})^{-1}$
8. $a \cdot 0 = 0$
Proof:
$a \cdot 0 = a(0 + 0)$ by F3
$= a \cdot 0 + a \cdot 0$ by F5
$= 0 = a \cdot 0$
9. $\forall b, 0 \cdot b \ne 1$
So there is no 0−1
10. $(-a) \cdot b = a \cdot (-b) = -(a \cdot b)$
11. $(-a) \cdot (-b) = a \cdot b$
12. (Bonus) $\,\! (a + b)(a - b) = a^2 - b^2$

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