Indexed Summation over Translated Interval

Theorem
Let $a$ and $b$ be integers.

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

Let $f : \left[{a \,.\,.\, b}\right] \to \R$ be a real-valued function.

Let $c\in\Z$ be an integer.

Then we have an equality of indexed summations:


 * $\displaystyle \sum_{i \mathop = a}^b f(i) = \sum_{i \mathop = a+c}^{b+c} f(i-c)$