Here is the problem
Suppose each of the numbers $a_1,\dots,a_n$ is either $1$ or $-1$ and $S=a_1a_2a_3a_4+a_2a_3a_4a_5+\dots+a_na_1a_2a_3=0$. Prove that $4|n$
Could someone help me with this problem?The Problem
from Hot Weekly Questions - Mathematics Stack Exchange
Itachi Uchiha
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