# Power Set of Group under Induced Operation is Semigroup/Proof 1

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## Theorem

Let $\struct {G, \circ}$ be a group.

Let $\struct {\powerset G, \circ_\mathcal P}$ be the algebraic structure consisting of the power set of $G$ and the operation induced on $\powerset G$ by $\circ$.

Then $\struct {\powerset G, \circ_\mathcal P}$ is a semigroup.

## Proof

We need to prove closure and associativity.

### Closure

Let $\struct {G, \circ}$ be a group, and let $A, B \subseteq G$.

\(\displaystyle \) | \(\) | \(\displaystyle \forall a \in A, b \in B: a \circ b \in G\) | |||||||||||

\(\displaystyle \) | \(\leadsto\) | \(\displaystyle A \circ B \subseteq G\) | Definition of Subset Product | ||||||||||

\(\displaystyle \) | \(\leadsto\) | \(\displaystyle A \circ B \in \powerset G\) | Definition of Power Set |

Thus $\struct {\powerset G, \circ_\PP}$ is closed.

$\Box$

### Associativity

It follows from Subset Product within Semigroup is Associative that $\circ_\PP$ is associative in $\struct {\powerset G, \circ_\PP}$.

$\Box$

Thus $\struct {\powerset G, \circ_\PP}$ is a semigroup.

$\blacksquare$

## Sources

- 1978: Thomas A. Whitelaw:
*An Introduction to Abstract Algebra*... (previous) ... (next): $\S 41.1$ Multiplication of subsets of a group