Preorders and their special cases

\[ \boxed { \begin{array} {} & \text {preorders} & \\ & \text {reflexive:} \; x \leq x & \\ & \text {transitive:}\; x\leq y \;\&\; y\leq z \Rightarrow x\leq z & \\ \boxed { \begin{array} {} \text {partial order} \\ \text {skeletal:}\; x\leq y \;\&\; y\leq x \Rightarrow x=y \\ \text {example: power set } X\mathcal P \\ \end{array} } & \boxed { \begin{array} {} \text{intersection} \\ \text{discrete order: only $x=x$ allowed } \\ \end{array} } & \boxed { \begin{array} {} \text {equivalence relation} \\ \text {symmetric: } x\leq y \Rightarrow y\leq x \\ \text{example: congruence } m \equiv n \; (\text{mod } k) \\ \end{array} }\\ \end{array}  } \]

Groups in linear algebra and elementary geometry

Preliminary Draft 

There are five groups that play an important role in linear algebra and elementary geometry.

The following table shows what they are and the relations between them.

Here $\bf k$ is an arbitrary commutative field (like $\R$ or $\bf C$).

\[ \boxed {\begin{array} {} \text {metric-preserving} && \text {not metric-preserving} \\ \hline \text {Isom}_n ({\bf k}) = \text {O}_n ({\bf k})  \ltimes \text {Trans}_n ({\bf k}) & \subset & \text {Aff}_n ({\bf k}) = \text {GL}_n ({\bf k})  \ltimes \text {Trans}_n ({\bf k}) \\  \cup && \cup \\ \text {O}_n ({\bf k})   & \subset & \text {GL}_n ({\bf k})  \end{array} } \]

The $\ltimes$ symbol denotes semi-direct product.

Reference: Andrew Baker, Matrix Groups