Partition of Indexing Set induces Bijection on Family of Sets/Lemma
Theorem
Let $I$ be an indexing set.
Let $\family {S_\alpha}_{\alpha \mathop \in I}$ be a family of sets indexed by $I$.
Let $I = I_1 \cup I_2$ such that $I_1 \cap I_2 = \O$.
Then there exists a bijection:
- $\ds \psi: \paren {\prod_{\alpha \mathop \in I_1} S_\alpha} \times \paren {\prod_{\alpha \mathop \in I_2} S_\alpha} \to \prod_{\alpha \mathop \in I} S_\alpha$
Proof
Let us define the mapping:
- $\ds \psi: \paren {\prod_{\alpha \mathop \in I_1} S_\alpha} \times \paren {\prod_{\alpha \mathop \in I_2} S_\alpha} \to \prod_{\alpha \mathop \in I} S_\alpha$
$\psi$ can be injective if and only if:
- $\map \psi a = \map \psi {a'} \implies a = a'$
where $\ds a, a' \in \paren {\prod_{\alpha \mathop \in I_1} S_\alpha} \times \paren {\prod_{\alpha \mathop \in I_2} S_\alpha}$
We can define a projection $\pr_1$:
- $\ds \pr_1: \paren {\prod_{\alpha \mathop \in I_1} S_\alpha} \times \paren {\prod_{\alpha \mathop \in I_2} S_\alpha} \to \prod_{\alpha \mathop \in I_1} S_\alpha$
so that for $\ds X \in \paren {\prod_{\alpha \mathop \in I_1} S_\alpha} \times \paren {\prod_{\alpha \mathop \in I_2} S_\alpha}, X = \tuple {X_1, X_2}$:
- $\map {\pr_1} X = X_1$
and similarly a projection $\pr_2$:
- $\ds \pr_2: \paren {\prod_{\alpha \mathop \in I_1} S_\alpha} \times \paren {\prod_{\alpha \mathop \in I_2} S_\alpha} \to \prod_{\alpha \mathop \in I_2} S_\alpha$
so that for $\ds X \in \paren {\prod_{\alpha \mathop \in I_1} S_\alpha} \times \paren {\prod_{\alpha \mathop \in I_2} S_\alpha}, X = \tuple {X_1, X_2}$:
- $\map {\pr_2} X = X_2$
From these we can build $\psi$ so that for $I_1 = \set {\alpha: \alpha \in I_1}$ and $I_2 = \set {\alpha: \alpha \in I_2}$:
- $\ds \map \psi {\mathbf x} = \paren {\prod_{\alpha \mathop \in I_1} \map {\psi_\alpha} {x_\alpha} } \times \paren {\prod_{\alpha \mathop \in I_2} \map {\psi_\alpha} {x_\alpha} }$
as $\psi$ uniquely sets all components of $\map {\pr_1} X$ and $\map {\pr_2} X$.
These in turn uniquely set the $2$ components of $X$.
That is:
- $\map f X = \map f {X'} \implies X = X'$
Surjection follows trivially.
$\blacksquare$
Sources
- 1975: Bert Mendelson: Introduction to Topology (3rd ed.) ... (previous) ... (next): Chapter $1$: Theory of Sets: $\S 10$: Arbitrary Products: Exercise $3$