# Divisors of Repunit with Composite Index

## Theorem

Let $R_n$ be a repunit number with $n$ digits.

Let $n$ be composite such that $n = r s$ where $1 < r < n$ and $1 < s < n$.

Then $R_r$ and $R_s$ are both divisors of $R_n$.

## Proof

Let $n = r s$.

Then:

 $\ds R_n$ $=$ $\ds \sum_{k \mathop = 0}^{n - 1} 10^k$ Basis Representation Theorem $\ds$ $=$ $\ds \sum_{j \mathop = 0}^{s - 1} \paren {\sum_{k \mathop = 0}^{r - 1} 10^k} 10^{r j}$ $\ds$ $=$ $\ds \paren {\sum_{k \mathop = 0}^{r - 1} 10^k} \paren {\sum_{j \mathop = 0}^{s - 1} 10^{r j} }$ $\ds$ $=$ $\ds R_r \paren {\sum_{j \mathop = 0}^{s - 1} 10^{r j} }$

Similarly:

$R_n = R_s \paren {\ds \sum_{j \mathop = 0}^{r - 1} 10^{s j} }$

$\blacksquare$

Thus, for example:

 $\ds R_6 = 111 \, 111$ $=$ $\ds 11 \times 10 \, 101$ $\ds$ $=$ $\ds 111 \times 1001$ $\ds R_{10} = 1 \, 111 \, 111 \, 111$ $=$ $\ds 11 \times 101 \, 010 \, 101$ $\ds$ $=$ $\ds 11111 \times 100 \, 001$

The pattern is clear.