Change of Variables in Summation over Finite Set

Theorem
Let $\mathbb A$ be one of the standard number systems $\N, \Z, \Q, \R, \C$.

Let $S$ and $T$ be finite sets.

Let $f: S \to \mathbb A$ be a mapping.

Let $g: T \to S$ be a bijection.

Then we have an equality of summations over finite sets:


 * $\ds \sum_{s \mathop \in S} \map f s = \sum_{t \mathop \in T} \map f {\map g t}$

Proof
Let $n$ be the cardinality of $S$ and $T$.

Let $\N_{<n}$ be an initial segment of the natural numbers.

Let $h : \N_{<n} \to T$ be a bijection.

By definition of summation:
 * $\ds \sum_{t \mathop \in T} \map f {\map g t} = \sum_{i \mathop = 0}^{n - 1} \map f {\map g {\map h i} }$

By Composite of Bijections is Bijection, the composition $g \circ h : \N_{<n} \to S$ is a bijection.

By definition of summation:
 * $\ds \sum_{s \mathop\in S} \map f s = \ds \sum_{i \mathop = 0}^{n - 1} \map f {\map g {\map h i} }$

Also see

 * Change of Variables in Indexed Summation
 * Finite Summation does not Change under Permutation