Structure is Group iff Semigroup and Quasigroup

Theorem
Let $\struct {S, \circ}$ be an algebraic structure.

Then:
 * $\struct {S, \circ}$ is a group


 * $\struct {S, \circ}$ is both a semigroup and a quasigroup.

Sufficient Condition
Let $\struct {S, \circ}$ be a group.

Then a fortiori $\struct {S, \circ}$ is a semigroup.

From Regular Representations in Group are Permutations:
 * for all $a \in S$, the left regular representation and the rightt regular representation are permutations of $S$.

Hence by definition $\struct {S, \circ}$ is a quasigroup.

Necessary Condition
Let $\struct {S, \circ}$ be both a semigroup and a quasigroup.

By definition of quasigroup:
 * $\forall a \in S$, the left and right regular representations $\lambda_a$ and $\rho_a$ are permutations on $S$.

It follows from Regular Representations in Semigroup are Permutations then Structure is Group that $\struct {S, \circ}$ is a group.