Question
Determine whether the following operations define a binary operation on the given set or not: $\ '\times _6\ '$ on $S = {1, 2, 3, 4, 5}$ defined by, $a \times _6 b =$ Remainder when $ab$ is divided by $6.$

Answer

Consider the composition table,
$\times _6$ $1$ $2$ $3$ $4$ $5$
$1$ $1$ $2$ $3$ $4$ $5$
$2$ $2$ $4$ $0$ $2$ $4$
$3$ $3$ $0$ $3$ $0$ $3$
$4$ $4$ $2$ $0$ $4$ $2$
$5$ $5$ $4$ $3$ $2$ $1$
Here all the elements of the table are not in $S.$
For $a = 2$ and $b = 3,$
$\text{a}\times_6\text{b}= 2\times_63 =$ remainder when $6$ divided by $6=0\neq\text{S}$
Thus, $\times _6$ is not a binary operation on $S.$

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