2015 USAMO Problems/Problem 5
Problem
Let be distinct positive integers such that
. Show that
is a composite number.
Solution
Note: This solution is definitely not what the folks at MAA intended, but it works!
Look at the statement . This can be viewed as a special case of [Conjecture], stating that the equation
has no solutions over positive integers for
and
. This special case was proven in 2009 by Michael Bennet, Jordan Ellenberg, and Nathan Ng, as
. This case
is obviously contained under that special case, so
and
must have a common factor greater than
.
Call the greatest common factor of and
. Then
for some
and likewise
for some
. Then consider the quantity
.
.
Because and
are both positive,
, and by definition
, so
is composite.
~BealsConjecture