Find all the triples of integers such that the number
is a power of .
(A power of is an integer of the form , where is a non-negative integer).
Solution
Let , , be integers and be a positive integer such that
We set , and we rewrite the equation as
If , then the right hand side is divisible by , so we have that
or
or
Note that, by Fermat's Little Theorem, for any integer the cubic residues are .
It follows that in (1) some of , and should be divisible by .
But in this case, is divisible by and this is a contradiction.
So, the only possibility is to have and consequently, , or, equivalently,
The solutions for this are ,
so the required triples are , , and all their cyclic permutations.
Alternative version:
If then divides , that is, the equation
has the solution , .
But then and have to be modulo , implying , which is a contradiction.
We can continue now as in the first version.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.