1986 IMO Problems/Problem 1

Problem

Let $d$ be any positive integer not equal to $2, 5$ or $13$ . Show that one can find distinct $a,b$ in the set $\{2,5,13,d\}$ such that $ab-1$ is not a perfect square.

Solution

Solution 1

We do casework with mods.

$d\equiv 0,3 \pmod{4}: 13d-1$ is not a perfect square.

$d\equiv 2\pmod{4}: 2d-1$ is not a perfect square.

Therefore, $d\equiv 1, \pmod{4}.$ Now consider $d\pmod{16}.$

$d\equiv 1,13 \pmod{16}: 13d-1$ is not a perfect square.

$d\equiv 5,9\pmod{16}: 5d-1$ is not a perfect square.

As we have covered all possible cases, we are done.

Solution 2

Proof by contradiction:

Suppose $p^2=2d-1$ , $q^2=5d-1$ and $r^2=13d-1$ . From the first equation, $p$ is an odd integer. Let $p=2k-1$ . We have $d=2k^2-2k+1$ , which is an odd integer. Then $q^2$ and $r^2$ must be even integers, denoted by $4n^2$ and $4m^2$ respectively, and thus $r^2-q^2=4m^2-4n^2=8d$ , from which $2d=m^2-n^2=(m+n)(m-n)$ can be deduced. Since $m^2-n^2$ is even, $m$ and $n$ must either be odd or even simultaneously, so $(m+n)(m-n)$ is divisible by $4$ . It follows that the odd integer $d$ must be divisible by $2$ , leading to a contradiction. We are done.

Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.

1986 IMO (Problems) • Resources
Preceded by First Problem	1 • 2 • 3 • 4 • 5 • 6	Followed by Problem 2
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1986 IMO Problems/Problem 1

Contents

Problem

Solution

Solution 1

Solution 2