2009 Indonesia MO Problems/Problem 2


For any real $x$, let $\lfloor x\rfloor$ be the largest integer that is not more than $x$. Given a sequence of positive integers $a_1,a_2,a_3,\ldots$ such that $a_1>1$ and \[\left\lfloor\frac{a_1+1}{a_2}\right\rfloor=\left\lfloor\frac{a_2+1}{a_3}\right\rfloor=\left\lfloor\frac{a_3+1}{a_4}\right\rfloor=\cdots\] Prove that \[\left\lfloor\frac{a_n+1}{a_{n+1}}\right\rfloor\leq1\] holds for every positive integer $n$.


By assuming $\left\lfloor\frac{a_n+1}{a_{n+1}}\right\rfloor >2$ we can conclude that: \[\left\lfloor\frac{a_1+1}{a_2}\right\rfloor=\left\lfloor\frac{a_2+1}{a_3}\right\rfloor=\left\lfloor\frac{a_3+1}{a_4}\right\rfloor=\cdots >2\]

Since $\left\lfloor\frac{a_1+1}{a_{2}}\right\rfloor >2$ , it is also true that $\frac{a_1+1}{a_{2}} > 2$, implying $a_1 > 2a_2 - 1 > a_2$.

After repeating this same process to all given fractions, we get: $a_1 > a_2 > a_3 > ...$ which is a impossible statement because $a_1, a_2, a_3, ...$ are all positive integers.

Therefore, $\left\lfloor\frac{a_n+1}{a_{n+1}}\right\rfloor\leq1$


See Also

2009 Indonesia MO (Problems)
Preceded by
First Problem
1 2 3 4 5 6 7 8 Followed by
Problem 3
All Indonesia MO Problems and Solutions