# Understand the problem

Prove that the positive integers \(n\) that cannot be written as a sum of \(r\) consecutive positive integers, with \(r>1\) ,are of the form \(n=2^l\) for some \(l\ge 0\).

##### Source of the problem

I.S.I. (Indian Statistical Institute) B.Stat/B.Math Entrance Examination 2019. Subjective Problem no. 1.

##### Topic

Number Theory

##### Difficulty Level

8.5 out of 10

##### Suggested Book

Elementary Number Theory by David M. Burton

# Start with hints

Do you really need a hint? Try it first!

Claim: Any positive integer \(n\) can be written as \(n=2^k\cdot m\) , where \(k\ge0\) and \(m\) is an odd positive integer.

To prove this claim use the fact : \(n=2^k\cdot p_1^{k_1}\cdot p_2^{k_2}\cdots p_i^{k_i}\), where \(k\) and all \(k_i\) are non-negetive integers and all \(p_i\) are odd primes.

The sum of (any) \(r\) consecutive positive integers is given by,

\((q+1)+(q+2)+(q+3)+\cdots+(q+r)\)

= \(q\cdot r+(1+2+3+\cdots+r)\)

= \(q\cdot r+\frac{r(r+1)}{2}\).

Equating this sum to \(n\) we get,

\(2^k\cdot m = q\cdot r+\frac{r(r+1)}{2}\)

Or, \(2^{k+1}\cdot m = 2q\cdot r+r(r+1)\)

Or, \(2^l\cdot m = r(2q+r+1)\) , where \(l=k+1\ge1\).

In particular if we take \(n=2^l\) then \(m\) is equal to 1.

Since both \(r\) and \((2q+r+1)\) are greater than 1, so they can’t be equal to \(m\) in this case. Again one of \(r, (2q+r+1)\) is odd integer which implies the product \(r(2q+r+1)\) can’t be equal to \(2^l\).

\(\Rightarrow 2^l \neq r(2q+r+1) \)

\(\Rightarrow 2^l\) can’t be expressed as the sum of \(r\) consecutive positive integers with \(r>1\) and \(l\ge 1\).

Now, \(n=2^0=1 \) also can’t be written in the same manner when \(l=0\). Therefore the positive integers \(n\) that cannot be written as a sum of \(r\) consecutive positive integers, with \(r>1\) , are of the form \(n=2^l\) for some \(l\ge 0\).

**To show that ****any number other than of the form \(2^l\) is sum of consecutive integers**:

Observe that for \(n=2^l.m\), where \(m\) is an odd number greater than 1, there are two cases:

1. **\( m < 2^l \)**

Select \(r\) and \(q\) such that \(r = m\) and \(2q+r+1 = 2^l \).

2. **\( m > 2^l \)**

Select \(r\) and \(q\) such that \( 2q+r+1 = m\) and \( r = 2^l \).

(QED).

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Very Good

Thank you. Keep doing great mathematics.

The proof is not yet complete.There may be such n which can’t be expressed as the sum of consecutive integers but also is not of the form 2^l. To disprove it we have to show that any number other than of the form 2^l is some of consecutive integers. How can we prove this?

Thanks, Alapan for your valuable suggestion. We have made the necessary changes. Keep doing great Mathematics. Hope it helps now.