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Noncommutative

From AoPSWiki

Informally, noncommutative means "order matters".

More formally, if $\star$ is some binary operation on a set, and $x$ and $y$ are elements of that set, then noncommutative means that $x \star y$ doesn't necessarily equal $y \star x$ .

Most common operations, such as addition and multiplication of numbers, are commutative. For example, $4\cdot3=3\cdot4=12$ , and $2+3=3+2=5$ .

Contents

1 Examples of noncommutative operations

Examples of noncommutative operations

Composition of functions

If $f(x)$ and $g(x)$ are functions, then usually, $(f\circ g)(x)\ne(g\circ f)(x)$ . This can also be written $g(f(x))\ne f(g(x))$ .

For example, suppose $f(x) = x^2$ and $g(x) = x+1$ . Then $(f\circ g)(x)=g(f(x))=g(x^2)=x^2+1$ , and $(g\circ f)(x)=f(g(x))=f(x+1)=(x+1)^2=x^2+2x+1$ . Unless $x=0$ , $(f\circ g)(x)$ will not be the same as $(g\circ f)(x)$ .

Matrix multiplication

If $A$ and $B$ are both $n\times n$ matrices, then usually, $AB\ne BA$ . For example:

$\begin{pmatrix}1&2\\3&4\end{pmatrix}\begin{pmatrix}5&6\\7&8\end{pmatrix}=\begin{pmatrix}19&22\\43&50\...$

whereas

$\begin{pmatrix}5&6\\7&8\end{pmatrix}\begin{pmatrix}1&2\\3&4\end{pmatrix}=\begin{pmatrix}23&34\\31&46\...$

Symmetries of a regular n-gon

The symmetries of a regular n-gon form a noncommutative group called a dihedral group.

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