Epimorphism Preserves Groups

Theorem
Let $\phi: \left({S, \circ}\right) \to \left({T, *}\right)$ be an epimorphism.

If $\left({S, \circ}\right)$ is a group, then so is $\left({T, *}\right)$.

Corollary
Isomorphism preserves groups.

Let $\phi: \left({S, \circ}\right) \to \left({T, *}\right)$ be an isomorphism.

If $\left({S, \circ}\right)$ is a group, then so is $\left({T, *}\right)$.

Proof

 * From Epimorphism Preserves Semigroups, if $\left({S, \circ}\right)$ is a semigroup then so is $\left({T, *}\right)$.


 * From Epimorphism Preserves Identity, if $\left({S, \circ}\right)$ has an identity $e_S$, then $\phi \left({e_S}\right)$ is the identity for $*$.


 * From Epimorphism Preserves Inverses, if $x^{-1}$ is an inverse of $x$ for $\circ$, then $\phi \left({x^{-1}}\right)$ is an inverse of $\phi \left({x}\right)$ for $*$.

The result follows from the definition of group.

Proof of Corollary
An isomorphism is an epimorphism.

Hence the result.