Embedding Theorem

== Motivation ==
== Theorem ==
Let:
:$(1): \quad \struct {T_2, \oplus_2}$ be a submagma of $\struct {S_2, *_2}$
:$(2): \quad f: \struct {T_1, \oplus_1} \to \struct {T_2, \oplus_2}$ be an isomorphism
then there exists:
:$(1): \quad$ a magma $\struct {S_1, *_1}$ which algebraically contains $\struct {T_1, \oplus_1}$
:$(2): \quad g: \struct {S_1, *_1} \to \struct {S_2, *_2}$ where $g$ is an isomorphism which extends $f$.
=== Corollary ===
== Proof ==
There are two cases to consider: when $T_1$ and $S_2$ are disjoint, and when they are not.
Suppose $T_1$ and $S_2$ are disjoint.
Let $S_1$ be the set $T_1 \cup \paren {S_2 \setminus T_2}$.
Then we can define the mapping $h: S_2 \to S_1$ as:
:$\forall x \in S_2: \map h x = \begin{cases}
x & : x \in S_2 \setminus T_2 \\
\map {f^{-1} } x & : x \in T_2
\end{cases}$
Because $T_1$ and $S_2$ are disjoint, $h$ can be seen to be a bijection.
What $h$ is doing is that it effectively "slots" $T_1$ into the gap in $S_2$ that was taken up by the removed $T_2$.
We are going to show that $\struct {T_1, \oplus_1}$ is embedded in $\struct {S_1, *_1}$.
The operation $*_1$ is defined as the transplant of $*_2$ under $h$.
Thus by the Transplanting Theorem:
:$\forall x, y \in S_1: x *_1 y = \map h {\map {h^{-1} } x *_2 \map {h^{-1} } y}$
Let $x, y \in T_1$.
Then, from the definition of $h$ above:
:$\map {h^{-1} } x = \paren {\map {f^{-1} } x}^{-1} = \map f x$
and similarly:
:$\map {h^{-1} } y = \map f y$
So:
proving that $x \oplus_1 y$ is closed.
Thus:
:$\struct {T_1, \oplus_1} \subseteq \struct {S_1, *_1}$
that is, $\struct {T_1, \oplus_1}$ is embedded in $\struct {S_1, *_1}$.
Now, let $g = h^{-1}$.
By the definition of $*_1$, $g$ is an isomorphism from $\struct {S_1, *_1}$ onto $\struct {S_2, *_2}$.
Then:
Thus:
So $g$ is an extension of $f$.
We have therefore proved that the embedding theorem holds when $T_1$ and $S_2$ are disjoint.
Next, suppose $T_1$ and $S_2$ are not disjoint.
We can use Exists Bijection to a Disjoint Set to assume the existence of a bijection $k: S_2 \to S_3$ such that $S_3 \cap T = \O$.
Let $*'$ be the transplant of $*$ under $k$.
Let:
:$T_3 = \set {\map k x: x \in T_2}$
Then:
So $T_3$ is closed under $*'$.
Let $*' {\restriction_{T_3} }$ be the operation on $T_3$ induced by $*'$.
Then $\struct {T_3, *' {\restriction_{T_3} } }$ is embedded in $\struct {S_3, *'}$.
Thus $k \circ f$ is an isomorphism from $\struct {T_1, \oplus_1}$ onto $\struct {T_3, *' {\restriction_{T_3} } }$.
$S_3$ has been constructed so as to be disjoint from $T_1$.
In such a case it has already been shown that the Embedding Theorem holds.
That is:
:$(1): \quad$ there exists a magma $\struct {S_1, *_1}$ containing $\struct {T_1, \oplus_1}$ algebraically
:$(2): \quad$ there exists an isomorphism $g_1$ from $\struct {S_1, *_1}$ to $\struct {S_3, \oplus_1}$ extending $k \circ f$
where $\circ$ denotes the composition of $k$ with $f$
Let $g = k^{-1} \circ g_1$.
Let $x \in T$.
Then:
and so $g$ extends $f$.
As $k$ is an isomorphism from $\struct {S_2, *_2}$ onto $\struct {S_3, *'}$, then:
:$(1): \quad g$ is an isomorphism from $\struct {S_1, *_1}$ onto $\struct {S_2, *_2}$
:$(2): \quad g$ extends $f$.
Hence the result.
== Sources ==
* : Chapter $\text {II}$: New Structures from Old: $\S 8$: Compositions Induced on Subsets: Theorem $8.1$
Category:Embedding Theorem
Category:Isomorphisms (Abstract Algebra)
Category:Named Theorems