In one of the Numberphile video – we can learn that 16 is a very special number because it is the only integer number which can be written as or .

But how do we show that this is actually true ? Well let us look for all integers and such that . Since we are not looking for the trivial solution for which let us impose .

It follows that . And since we know that is an integer, so that is an integer and therefore is an integer too.

In other words for some integer . Replacing by in the equation we started with we get

which leads to

or .

From here you can:

1 – observe that is always smaller than expect for

2 – observe that is never integer but for

both impose and , which ends the proof.

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