# Practice Paper 4 Question 15

In base 10, or decimal, we use the digits from 0 to 9 to represent any positive integer. In base 2, or binary, we use the digits from 0 to 1 to do the same. An integer is called a *repunit* (repeated unit) if it can be written in some base using only the digit 1. Find all \(n,p>0\) such that a binary repunit with \(n\) digits is equal (in value) to a decimal repunit with \(p\) digits. Prove your answer.

## Related topics

## Warm-up Questions

- What is the value of \(2222_3\) in base \(10?\)
- Let \(a\) be a number in base \(b\) with digits \(a_na_{n-1}\ldots a_0.\) Show that \(a \equiv a_0 \mod b.\)

## Hints

- Hint 1Which values of \(n\) and \(p\) work and which don't?
- Hint 2Are there any constraints relating \(n\) and \(p?\)
- Hint 3More specifically, pay attention to the number of digits used to express a number in base \(2\) compared to in base \(10.\)
- Hint 4You should have found that \(n=p=1\) is a solution. If there are other solutions, could you try to establish a lower bound on \(p?\)
- Hint 5Are any other solutions? Could you prove your hypothesis?
- Hint 6How can you mathematically express that
*"a binary repunit with \(n\) digits is equal in value to a decimal repunit with \(p\) digits"*? - Hint 7... as an equality of two geometric series?
- Hint 8Have you tried simplifying and rearranging the expression into a form which you can apply your constraints to get a contradiction?

## Solution

We can easily see that \(n=p=1\) works. What about other values? We can also see that \(n>p,\) since any number greater than \(1\) needs more binary digits than base \(10\) digits to be represented.

By inspection it's also clear that \(p=2,3\) would not work since those are the decimal repunits \(11\) and \(111,\) neither of which are equal to any binary repunits. The closest are \(111_2=8\) and \(1111111_2=127.\) Therefore \(p>3.\)

The original condition can be written as \(\sum_{i=0}^{n-1}2^i=\sum_{i=0}^{p-1}10^i,\) which gives \(2^n-1=\) \({(10^p-1) \over 9}.\) We can rearrange this to get \(9\cdot2^{n-p}-5^p=2^{3-p}.\)

Together with the above conditions, \(n>p\) and \(p>3,\) we get a contradiction, since \(2^{3-p}\) is a rational less than 1 and \(9\cdot2^{n-p}-5^p\) is an integer. Therefore \(n=p=1\) is the only solution.

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