Indexed Summation of Sum of Mappings

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

Let $a,b$ be integers.

Let $\left[{a \,.\,.\, b}\right]$ denote the integer interval between $a$ and $b$.

Let $f,g : \left[{a \,.\,.\, b}\right] \to \mathbb A$ be mappings.

Let $h=f+g$ be their pointwise addition.

Then we have the equality of indexed summations:
 * $\displaystyle \sum_{i \mathop = a}^b h(i) = \sum_{i \mathop = a}^b f(i) + \sum_{i \mathop = a}^b g(i)$

Proof
The proof goes by induction on $b$.

Basis for the Induction
Let $b < a$.

Then all indexed summations are zero.

Because $0 = 0 + 0$, the result follows.

This is our basis for the induction.

Induction Step
Let $b \geq a$.

We have:

By the Principle of Mathematical Induction, the proof is complete.

Also see

 * Indexed Summation of Multiple of Mapping
 * Summation of Sum of Mappings on Finite Set