This post is inspired by a tweet from @standupmaths who sends out puzzles tagged #mathspuzzle irregularly but fairly frequently. I won’t put out any spoilers on the blog, but once the solution is out on twitter, I may occasionally muse on these here. It’s my first go at both maths blogging and embedding equations into a webpage, so any comments welcome. I’m pretty sure I’ve gone round the houses on this one, so shortened method / notation welcome (as well as pointing out any errors that I’ve made, obviously).

The problem was stated thus: “#MathsPuzzle: What is the only number 10 →20 that is not a sum of consecutive numbers? (eg. Not 12 because 3 + 4 + 5 = 12)”. Tweeters fairly quickly answered 16 and the follow up question (abbreviated) was – how does this generalise? To which the answer is the only positive integers that cannot be represented as the sum of consecutive integers are the set \(x \in \{2^n\}\) where n is an integer.

After finding this result, I tried to express why \(\{2^n\}\) *and only* \(\{2^n\}\) exhibit this property in 140 characters and failed miserably. So, I decided to blog about it and write the method I followed in my head down – if only to clarify to myself how I came to the result (and whether it was valid).

I began by using the result:

\[\begin{aligned} \sum_{i=1}^{i=j}{i} = \frac {j \cdot (j+1)}{2} \end{aligned} \]

therefore, the sum of integers between j and k is:

\[\begin{aligned} \sum_{i=1}^{i=k}{i} \quad – \quad \sum_{i=1}^{i=j}{i} \qquad & = \qquad \frac {k \cdot (k+1)}{2} – \frac {j \cdot (j+1)}{2} \\ & = \qquad \frac {k^2 – j^2 + k – j}{2} \\ & = \qquad \frac {(k+j) \cdot (k – j) + (k – j)}{2} \\ & = \qquad \frac {(k+j+1) \cdot (k-j)}{2} \end{aligned} \]

therefore, if x is to be expressed as the sum of some set of consecutive integers, the following must hold:

\[\begin{aligned} x \qquad &= \qquad \frac {(k+j+1) \cdot (k-j)}{2} \\ 2x \qquad &= \qquad{(k+j+1) \cdot (k-j)} \qquad (1)\end{aligned} \]

We notice that if *both* \(k\) and \(j\) are odd, or *both* \(k\) and \(j\) are even, then both \(k + j\) and \(k – j\) are even, therefore \((k+j+1)\) is odd and \((k-j)\) is even. Conversely, if one of \(k\) or \(j\) is even and the other odd, both \((k + j)\) and \((k – j)\) are odd \((k+j+1)\) is even and \((k-j)\) is odd.

So we may state that **if \(x\) is the sum of consecutive integers then we must be able to express \(2x\) as the product of one even and one odd integer**

If we next consider any integer as a product of primes, we can say that for any integer

\begin{equation}\label{primefactors} i = 2^a \cdot 3^b \cdot 5^c \cdot 7^d \cdots \qquad (2)\end{equation} and so on for all primes.

If we set \(i = 2x \) then we can combine (1) and (2) to see that for x to be the sum of consecutive integers,

\[\begin{aligned} 2x \qquad &= \qquad {(k+j+1) \cdot (k-j)} \\&= \qquad 2^a \cdot 3^b \cdot 5^c \cdot 7^d \cdots \\ \\ \text{therefore} \\ \\{(k+j+1) \cdot (k-j)}&= \qquad 2^a \cdot 3^b \cdot 5^c \cdot 7^d \cdots \qquad \qquad (3) \end{aligned} \]

As all primes are odd except 2, and \((\text{odd}\times\text{odd})\) is always odd, we can say that the right hand side of (3) will always be \((\text{even}\times\text{odd})\) except in the case where \( b = c = d = \cdots = 0 \) where it will be \( 2^a \). However, for \(x\) to be the sum of consecutive digits, this expression is required to be \((\text{even}\times\text{odd})\), which holds in all cases *except* where \(2x = 2^a \rightarrow x = 2^{a-1} \rightarrow x \in \{2^n\} \)