# 09-240/Classnotes for Thursday September 24

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 Convention for today: $x, y, z \in \mathbf V$ $a, b, c \in F$

A V.S. over $F: V, 0, +, \cdot$ s.t.

VS1 $\forall x, y \in \mathbf V, x + y = y + x$
VS2 $\ldots (x + y) + z = x + (y + z)$
VS3 $\ldots x + 0 = x$
VS4 $\forall x, \exists y \mbox{ s.t. } x + y = 0$
VS5 $1 \cdot x = x$
VS6 $a \cdot (b \cdot x) = (a \cdot b) \cdot x$
VS7 $a \cdot (x + y) = ax + ay$
VS8 $(a + b) \cdot x = ax + bx$

### Examples

1. $\left\{ \begin{pmatrix} a_1 \\ \vdots \\ a_n \end{pmatrix} : a_i \in F \right\} = F^n$
2. $\left\{ \begin{pmatrix} a_{11} & \cdots & a_{1n} \\ \vdots & \ddots & \vdots \\ a_{m1} & \cdots & a_{mn} \end{pmatrix} \right\} = \mathrm M_{m \times n}(F)$
3. Let S be a set (F is some field)
$\mathcal F(S, F) = \{f: S \rightarrow F\}$
S = Primary colours = {red, green, blue}
F = F2 = {0, 1}
$\mathcal F(S, F) = \left\{ \begin{matrix} f_1(red) = 0 & f_1(green) = 1 & f_1(blue) = 0 \\ \cdots \\ f_2 \begin{pmatrix} \mbox{red} \\ \mbox{green} \\ \mbox{blue} \end{pmatrix} = \begin{pmatrix} 0 \\ 1 \\ 0 \end{pmatrix} & \cdots \end{matrix} \right\}$

$S = \mathbb N = \{ 1, 2, 3, 4, \ldots \} F = \mathbb R$

$\mathcal F(\mathbb N, \mathbb R) = \left\{ \begin{matrix} 1 & 2 & 3 & 4 & \ldots \\ 6 & 6 & 6 & 6 & \ldots \\ \pi & 2\pi & e & 62 & \ldots \\ \end{matrix} \right\} = \{ \mbox{sequences} \}$

$S = \begin{pmatrix} \vdots \\ \vdots \end{pmatrix} n \Rightarrow \mathcal F(S, F) = F^n$

1. $O_{\mathcal F(S, F)}(\sigma) = 0_F \forall \sigma \in S$
2. $f, g \in \mathcal F(S, F)$
$\,\! (f + g)(\sigma) = f(\sigma) + g(\sigma)$
$f \in \mathcal F(S, F)$
$a \in F \Rightarrow \forall \sigma \in S, S(af)(\sigma) = a \cdot (f(\sigma))$

Claim: + is associative. Given $f, g, h \in \mathcal F(S, F), (f + g) + h = f + (g + h) \forall \sigma$

((f + g) + h)(σ) = (f + g)(σ) + h(σ)
= (f(σ) + g(σ)) + h(σ)
= f(σ) + g(σ) + h(σ) (by F2)
(f + (g + h))(σ) = f(σ) + (g + h)(σ)
= f(σ) + (g(σ) + h(σ))
= f(σ) + g(σ) + h(σ)

1. $\mathbb C \mbox{ is a V.S. over } \mathbb R$
2. $\mathbb R \mbox{ is a V.S. over } \mathbb Q$
3. $\mathbb R \mbox{ is a V.S. over } \mathbb R$
4. $\,\! \{0\} \mbox{ is a V.S. over } F$

### Dull theorem

1. Cancellation: $x + y = x + z \Rightarrow y = z$ (add w to both sides s.t. x + w = 0)
2. 0 is unique
3. Negatives are unique: $x + y = 0 = x + z \Rightarrow y = z$
4. $0x = 0. a \cdot 0 = 0$
5. $(-a) \cdot x = a \cdot (-x) = -(ax)$