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Friday, February 3, 2017

Cyclic Equations

For an integer n > 2, we have real numbers a_1, a_2, \cdots, a_n such that (here with n > 4 for clarity):

\begin{align*} a_2 + a_3 + a_4 +\cdots + a_n &= a_1 \\ a_1 + a_3 + a_4 + \cdots + a_n &= a_2 \\ &\vdots \\ a_1 + a_2 + a_3 + \cdots + a_{n-1} &= a_n \end{align*}

(In other words, a_i = \Sigma_{\{j \mid 1 \le j \le n, i \ne j\}} a_j)

What is the value of a_1 + a_2 + a_3 + \cdots + a_n?
(Source: Brilliant (problem page))

This one is as straightforward as adding all the equations together, and seeing what happens. The result will be the equation (n - 1)(a_1 + a_2 + a_3 + \cdots + a_n) = a_1 + a_2 + a_3 + \cdots + a_n. (Try it at n=3 and n=4 for simple concrete examples, if needed.) Whenever, as stated, n > 2, the sum of the variables can only ever be zero.

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