# A Russian puzzle

Dave Richeson tweeted about a puzzle from Futility Closet (original source a Russian mathematical olympiad): can you split the integers 1, 2, …, 15 into two groups A and B, with 13 elements in A and 2 elements in B, so that the sum of the elements of A is the product of the elements of B?

Think about it for a moment. There’s of course the temptation to brute-force it, which is doable, but there’s a more elegant solution.

This got me thinking – when can you split the integers 1, 2, …, n into two groups A and B, where B has two elements, so that the sum of the elements of A is the product of the elements of B?

Say B contains x and y. Then their product is of course xy. The sum of the elements of A is $1 + 2 + ... + n - (x+y) = n(n+1)/2 - (x+y)$. Setting these equal and rearranging gives

$n(n+1)/2 + 1 = xy + x + y + 1$

where we’ve added 1 to make the factorization work out – this becomes

$n(n+1)/2 + 1 = (x+1)(y+1)$.

So the problem is reduced to finding factorizations of n(n+1)/2 + 1, which satisfy two conditions:

• x and y can’t be equal (for specificity we’ll say x < y), and
• x and y are both at most n.

Since we have y ≤ n, we’re going to have x ≥ (n+1)/2 + 1/n. n must be at least 2, so we can just write x ≥ (n/2) + 1. So we’re looking for factors of n(n+1)/2 + 1 in the interval [n/2+1, n]. Here’s some brute-force Python code to find all such solutions:


import math

def solutions(n):
out = []
total = n*(n+1)/2+1
xmin = int(math.ceil(n/2.0) + 1)
xmax = n
for x in range(xmin, xmax+1):
if total % (x+1) == 0:
y = total/(x+1)-1
if x < y:
out.append([x,y])
return out

def all_solutions(n):
out = []
for i in range(2, n+1):
sols = solutions(i)
for sol in sols:
sol.insert(0, i)
out.append(sol)
return out


solutions takes an integer n as input and returns pairs [x, y] which are solutions to the problem. For example solutions(17) returns [[10, 13]].
And all_solutions takes an integer N and returns all triples [n, x, y] with $n \le N$ which are solutions to the problem — that is, where xy equals the sum of all the integers up to n except for x and y. The first few solutions are:

 n x y 10 6 7 17 10 13 26 15 21 36 22 28 37 21 31 45 27 36 50 28 43 61 42 43 65 36 57 67 42 52 78 45 66 82 45 73 91 52 78 94 57 76 101 55 91 102 70 73 110 70 85 122 66 111 136 76 120 138 87 108

So it appears that there’s nothing particularly special about the number 15 in the initial puzzle. There are plenty of values n for which you can’t do this, and plenty for which you can. Also, there are values of n for which there are multiple solution pairs (x, y), although not surprisingly they are rare. The smallest such n is 325, for which $x = 171, y = 307$ and $x = 175, y = 300$ are both solutions. In this case $latex n(n+1)/2 + 1 = 52976 = 24 \times 7 \times 11 \times 43$, from which 52976 has (5)(2)(2)(2) = 40 factors. A typical number of this size has about $log(52976) \approx 11$ factors. This abundance of factors makes it more likely that 52976 would have two factorizations of the sort we’re looking for. And in fact $52976 = 172 \times 308 = 176 \times 301$.

Solutions to this problem appear to have some interesting statistical properties… more on that in a future post.