# Sum of Indices of Real Number/Positive Integers

## 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:

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

$\blacksquare$