Extension Theorem for Homomorphisms

Theorem
Let the following conditions be fulfilled:


 * Let $$\left({S, \circ}\right)$$ be a commutative semigroup with cancellable elements;
 * Let $$\left({C, \circ}\right) \subseteq \left({S, \circ}\right)$$ be the subsemigroup of all cancellable elements of $$S$$;
 * Let $$\left({S', \circ'}\right)$$ be an inverse completion of $$\left({S, \circ}\right)$$;
 * Let $$\phi$$ be a homomorphism from $$\left({S, \circ}\right)$$ into a semigroup $$\left({T, *}\right)$$ such that $$\phi \left({y}\right)$$ is invertible for all $$y \in C$$.

Then:
 * 1) There is one and only one homomorphism $$\phi'$$ from $$\left({S', \circ'}\right)$$ into $$\left({T, *}\right)$$ extending $$\phi$$;
 * 2) $$\phi' \left({x \circ' y^{-1}}\right) = \phi \left({x}\right) * \left({\phi \left({y}\right)}\right)^{-1}$$;
 * 3) If $$\phi$$ is a monomorphism, then so is $$\phi'$$.

Proof
(It is proved that $$\left({C, \circ}\right)$$ is a subsemigroup of $$\left({S, \circ}\right)$$ by Cancellable Elements of a Semigroup.)


 * To show there is at most one such homomorphism:

Let $$\psi$$ be a homomorphism from $$\left({S', \circ'}\right)$$ into $$\left({T, *}\right)$$ extending $$\phi$$.

Now $$\phi \left({y}\right)$$ is invertible and hence cancellable for $$*$$ by Invertible also Cancellable. So:

$$\forall x \in S, y \in C: \psi \left({x \circ' y^{-1}}\right) = \phi \left({x}\right) * \left({\phi \left({y}\right)}\right)^{-1}$$ by the morphism property.

From the definition of inverse completion, we have $$T = S \circ' C^{-1}$$.

So, there is at most one homomorphism extending $$\phi$$.

By Epimorphism Preserves Semigroups and Epimorphism Preserves Commutativity, as $$\left({S, \circ}\right)$$ is a commutative semigroup, then so is $$\phi \left({S}\right)$$, and is a subsemigroup of $$\left({T, *}\right)$$.

By Commutation with Inverses, every element of $$\phi \left({S}\right)$$ commutes with every element of $$\left({\phi\left({C}\right)}\right)^{-1}$$.


 * Thus we can verify that the function $$\phi': S' \to T$$ defined by:

$$\forall x \in S, y \in C: \phi' \left({x \circ' y^{-1}}\right) = \phi \left({x}\right) * \left({\phi \left({y}\right)}\right)^{-1}$$

is a well-defined homomorphism extending $$\phi$$, and that $$\phi'$$ is a monomorphism if $$\phi$$ is.