Indexed Summation over Interval of Length Two
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Theorem
Let $\mathbb A$ be one of the standard number systems $\N, \Z, \Q, \R, \C$.
Let $a \in \Z$ be an integer.
Let $f: \set {a, a + 1} \to \mathbb A$ be a real-valued function.
Then the indexed summation:
- $\ds \sum_{i \mathop = a}^{a + 1} \map f i = \map f a + \map f {a + 1}$
Proof
We have:
\(\ds \sum_{i \mathop = a}^{a + 1} \map f i\) | \(=\) | \(\ds \sum_{i \mathop = a}^a \map f i + \map f {a + 1}\) | Definition of Indexed Summation | |||||||||||
\(\ds \) | \(=\) | \(\ds \map f a + \map f {a + 1}\) | Indexed Summation over Interval of Length One |
$\blacksquare$