2012 Indonesia MO Problems/Problem 2

Problem

Show that for any positive integers $a$ and $b$ , the number $n=\mathrm{LCM}(a,b)+\mathrm{GCD}(a,b)-a-b$ is an even non-negative integer.

Solution

By AM-GM $(1+a_k)^k=(\frac{1}{k-1}+\frac{1}{k-1}+\dots+\frac{1}{k-1}+n_k)^k\geq (k\sqrt[k]{\frac{a_k}{(k-1)^{k-1}}})^k=\frac{k^k\cdot a_k}{(k-1)^{k-1}}$ Substituting back into our original equation we get $(1+a_2)^2(1+a_3)^3\dots(1+a_n)^n\geq \frac{2^2\cdot a_2}{1^1}\frac{3^3\cdot a_3}{2^2}\dots\frac{n^n\cdot a_n}{(n-1)^{n-1}}=\frac{2^2}{1^2}\frac{3^3}{2^2}\frac{4^4}{3^3}\dots\frac{(n-1)^{n-1}}{(n-2)^{n-2}}\frac{n^n}{(n-1)^{n-1}}\cdot a_1a_2a_3\dots a_n=n^n\cdot 1=n^n$ however, we only proved its $\geq n^n$ , for the equality to happen, $a_k=\frac{1}{k-1}$ for all $k$ , which is impossible for all of them to be so, thus the equality is impossible

2012 Indonesia MO (Problems)
Preceded by First Problem	1 • 2 • 3 • 4 • 5 • 6 • 7 • 8	Followed by Problem 2
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2012 Indonesia MO Problems/Problem 2

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