# Alternating sums of numbers divisible by $7$

Let $x_1,x_2,x_3,x_4,x_5,x_6$ be given integers, not divisible by $7$. Prove that at least one of the expressions of the form $$\pm x_1\pm x_2\pm x_3\pm x_4\pm x_5\pm x_6$$ is divisible by $7$, where the signs are selected in all possible ways. (Generalize the statement to every prime number greater than two!)

Each term is in $\{1,2,3,4,5,6\}$ modulo $7$, but how do I use this to prove the result?

• Are you assuming that the $x_i$ span the non-zero residues? If so, see this question – lulu Jun 7 '16 at 14:16
• Of course, for that case you could just pair each element with its additive inverse...so I suppose the problem is only interesting if the $x_i$ don't span the residues. – lulu Jun 7 '16 at 14:31
• use $\{-3,-2,-1,1,2,3\}$ as your set of residues instead of 1-6. Also, note that the same sums can be obtained from $|x_1|,...,|x_6|$ as can be obtained from $x_1, ..., x_6$, so you only need to consider terms in $\{1,2,3\}$. – Paul Sinclair Jun 7 '16 at 16:56
• Follows very easily from Cauchy-Davenport: mathworld.wolfram.com/Cauchy-DavenportTheorem.html – Erick Wong Jun 8 '16 at 0:45
• @ErickWong: Maybe consider making your comment an answer, so that it can be accepted and this question can exit the queue? – Kieren MacMillan May 27 '17 at 11:06

We make the simple observation that an alternating sum $\pm x_1 \pm \cdots \pm x_6$ is $0$ iff twice the sum of the positive terms is equal to $x_1 + \cdots + x_6$. This principle also works modulo $7$, so the claim holds if we know that some subset of $\{x_1,\ldots,x_6\}$ sums to a number congruent to $2^{-1}(x_1+\cdots+x_6)$ mod $7$. Cauchy-Davenport assures us that in fact every possible residue is covered.
This generalizes completely to any prime except for $2$ (since we use the fact that $2$ has a multiplicative inverse). For $p=2$ we only have one term (and the $\pm$ signs have no effect) so it the analogous claim is false.