Condition for Mapping between Structure and Cartesian Product of Substructures to be Bijection

Theorem
Let $\struct {S, \circ}$ be an algebraic structure with $1$ operation.

Let $\struct {A, \circ {\restriction_A} }$ and $\struct {B, \circ {\restriction_B} }$ be closed algebraic substructures of $\struct {S, \circ}$, where $\circ {\restriction_A}$ and $\circ {\restriction_B}$ are the operations induced by the restrictions of $\circ$ to $A$ and $B$ respectively.

Let the mapping $\phi: A \times B \to S$ be defined as:


 * $\forall \tuple {a, b} \in A \times B: \map \phi {a, b} = a \circ b$

where $A \times B$ denotes the Cartesian product of $A$ and $B$.

Then:
 * $\phi$ is a bijection


 * for all $s \in S$: there exists a unique $\tuple {a, b} \in A \times B$ such that $a \circ b = s$.
 * for all $s \in S$: there exists a unique $\tuple {a, b} \in A \times B$ such that $a \circ b = s$.

Proof
First we establish that from Set of Finite Subsets under Induced Operation is Closed:
 * $A \times B \subseteq S$

Thus:
 * $\forall \tuple {a, b} \in A \times B: \exists s \in S: s = a \circ b = \map \phi {a, b}$

Thus $\phi$ is indeed a mapping.

Sufficient Condition
Let $\phi$ be a bijection.

As $\phi$ is a bijection, it is surjective, so:


 * $S \subseteq A \times B$

Thus:
 * $S = A \times B$

That is, every element of $S$ is an element of $A \times B$.

Thus, every element of $S$ can be expressed as a product of the form $a \circ b$ such that $\tuple {a, b} \in A \times B$ in at least one way.

As $\phi$ is a bijection, it is injective.

Let $\map \phi {a_1, b_1} = \map \phi {a_2, b_2}$.

Then by definition of injective:
 * $\tuple {a_1, b_1} = \tuple {a_2, b_2}$

and thus:
 * $a_1 = a_2, b_1 = b_2$

From the definition of $\phi$, this means:
 * $a_1 \circ b_1 = a_2 \circ b_2$

Thus, every element of $S$ that can be expressed as a product of the form $a \circ b$ such that $\tuple {a, b} \in A \times B$ can be thus expressed uniquely.

In summary:
 * $\forall s \in S: \exists_1 \tuple {a, b} \in A \times B: s = a \circ b$

where $\exists_1$ denotes unique existence.

Necessary Condition
Let it be the case that:


 * for all $s \in S$: there exists a unique $\tuple {a, b} \in A \times B$ such that $a \circ b = s$.

Hence by definition of $\phi$:
 * $\forall s \in S: \exists \tuple {a, b} \in A \times B: s = \map \phi {a, b}$

That is, $\phi$ is surjective.

Now let $\tuple {a, b}$ and $\tuple {c, d}$ in $A \times B$ such that:
 * $\map \phi {a, b} = \map \phi {c, d}$

That is:
 * $\exists s \in S: s = a \circ b = c \circ d$

But such a representation of $s$ is unique.

That is:
 * $\tuple {a, b} = \tuple {c, d}$

and so $\phi$ is by definition an injection.

Thus $\phi$ is both a surjection and an injection.

Hence the result by definition of bijection.