# Sum of Indices of Real Number/Positive Integers

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## Contents

## Theorem

Let $r \in \R_{> 0}$ be a positive real number.

Let $n, m \in \Z_{\ge 0}$ be positive integers.

Let $r^n$ be defined as $r$ to the power of $n$.

Then:

- $r^{n + m} = r^n \times r^m$

## Proof

Proof by induction on $m$:

For all $m \in \Z_{\ge 0}$, let $\map P m$ be the proposition:

- $\forall n \in \Z_{\ge 0}: r^{n + m} = r^n \times r^m$

$\map P 0$ is true, as this just says:

- $r^{n + 0} = r^n = r^n \times 1 = r^n \times r^0$

### Basis for the Induction

$\map P 1$ is true, by definition of power to an integer:

- $r^{n + 1} = r^n \times r = r^n \times r^1$

This is our basis for the induction.

### Induction Hypothesis

Now we need to show that, if $\map P k$ is true, where $k \ge 1$, then it logically follows that $\map P {k + 1}$ is true.

So this is our induction hypothesis:

- $\forall n \in \Z_{\ge 0}: r^{n + k} = r^n \times r^k$

Then we need to show:

- $\forall n \in \Z_{\ge 0}: r^{n + k + 1} = r^n \times r^{k + 1}$

### Induction Step

This is our induction step:

\(\displaystyle r^n \times r^{k + 1}\) | \(=\) | \(\displaystyle r^n \times \paren {r^k \times r}\) | Definition of Integer Power | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \paren {r^n \times r^k} \times r\) | Real Multiplication is Associative | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle r^{n + k} \times r\) | Induction Hypothesis | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle r^{n + k + 1}\) | Definition of Integer Power |

So $\map P k \implies \map P {k + 1}$ and the result follows by the Principle of Mathematical Induction.

Therefore:

- $\forall n, m \in \Z_{\ge 0}: r^{n + m} = r^n \times r^m$

$\blacksquare$

## Sources

- 1966: Richard A. Dean:
*Elements of Abstract Algebra*... (previous) ... (next): $\S 0.1$: Example $1: \ \text{I}$