1996 OIM Problems/Problem 4

Problem

Given a natural number $n \ge 2$, consider all fractions of the form $\frac{1}{ab}$, where $a$ and $b$ are natural numbers, prime to each other and such that

\[a < b \le n\]

\[a + b > n\]

Prove that for each $n$ the sum of these fractions is 1/2.

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Solution

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See also

https://www.oma.org.ar/enunciados/ibe11.htm