Associative Operation/Examples

Examples of Associative Operations

Let $\struct {S, \circ}$ be an algebraic structure where $\circ$ is an associative operation.

Let $a \in S$ be an arbitrary element of $S$.

Let $*$ be the operation defined on $S$ by:

$\forall x, y \in S: x * y := x \circ a \circ y$

Then $*$ is associative on $S$.

$\begin{array}{c|cccc}

\circ & a & b & c \\ \hline a & b & c & b \\ b & b & a & c \\ c & a & c & c \\ \end{array}$

$\ds \paren {a \circ a} \circ b$

$=$

$\ds b \circ b$

$\ds $

$=$

$\ds a$

$\ds a \circ \paren {a \circ b}$

$=$

$\ds a \circ c$

$\ds $

$=$

$\ds b$

demonstrating non-associativity.

Also note that $a \circ b \ne b \circ a$, so $\circ$ is non-commutative as well.

Let $\R$ denote the set of real numbers.

Let $\circ$ denote the operation on $\R$ defined as:

$\forall x, y \in \R: x \circ y := x y + 1$

Then $\circ$ is not an associative operation, despite being commutative.