Product of Sums

Theorem
If $$\sum_{n\in A} a_n \ $$ and $$\sum_{n\in B} b_n \ $$ are absolutely convergent, then:
 * $$\left({ \sum_{i\in A} a_i }\right) \left({ \sum_{j\in B} b_j }\right) = \sum_{(i,j)\in A \times B} a_i b_j \ $$.

Corollary
If $$\sum_{i \in X} a_{ij} \ $$ is absolutely convergent $$\forall j\in Y \ $$, then:
 * $$\prod_{j \in Y} \left( \sum_{i \in X} a_{ij} \right) = \sum_{\sigma : Y \to Y} \left( \prod_{(i,j) \in X \times Y} a_{i \sigma(j)} \right) \ $$

where $$\sigma $$ runs over all bijections (permutations in the finite case) from $$Y$$ to $$Y$$.

Proof
Since both series are absolutely convergent, it is permitted to expand the product as:
 * $$\left({ \sum_{i\in A} a_i }\right) \left({ \sum_{j\in B} b_j }\right) = \sum_{i\in A} \left({ a_i  \sum_{j\in B} b_j}\right) \ $$.

But since $$a_i \ $$ is a constant, it may be brought into the sum:
 * $$ = \sum_{i\in A} \sum_{j\in B} a_i b_j  \ $$

which is precisely the theorem.

Proof of Corollary
We will prove the case $$X = Y = \N \ $$ to avoid the notational inconvenience of enumerating the elements of $$Y \ $$ as $$j_1, j_2, j_3 \dots \ $$. The general case where $$X, Y \ $$ are arbitrary sets has the same proof, but with more indices and notational distractions.

Consider that by the main theorem:
 * $$\prod_{j=1,2} \left({ \sum_{i \in \N} a_{ij} }\right) = \sum_{x,y\in \N} a_{x_1}a_{y_2} \ $$

and continuing in this vein:
 * $$\prod_{j=1,2,3} \left({ \sum_{i \in \N} a_{ij} }\right) = \left({ \sum_{x,y\in \N} a_{x_1}a_{y_2} }\right) \left({ \sum_{z\in \N} a_{z_3} }\right) = \sum_{x,y,z\in \N} a_{x_1}a_{y_2}a_{z_3} \ $$

For an inductive proof of this concept for finite $$n \ $$, we assume that for some $$n \in \N \ $$:
 * $$\prod_{j=1}^n \left({ \sum_{i \in \N} a_{ij} }\right) = \sum_{u,v,\dots,x,y \in \N} a_{u_1}a_{v_2}\dots a_{x_{(n-1)}}a_{y_n} \ $$

Then:
 * $$\prod_{j=1}^{n+1} \left({ \sum_{i \in \N} a_{ij} }\right) = \left({ \sum_{u,v,\dots,x,y \in \N} a_{u_1}a_{v_2}\dots a_{x_{(n-1)}}a_{y_n} }\right) \left({ \sum_{z\in \N} a_{z_n} }\right) \ $$

which by the main theorem is simply:
 * $$= \sum_{u,v,\dots,x,y,z \in \N} a_{u_1}a_{v_2}\dots a_{x_{(n-1)}}a_{y_n}a_{z_{(n+1)}} \ $$

completing the induction for finite $$n \ $$.