# Using your Head is Permitted

## December 2009 solution

#### Part 1:

I'm using here a proof sent to me by James Ge.
Let us assume on the contrary.

First, note that both sets need to have infinite size. If one has a finite
size, choose the smallest two elements from the other set that are larger than
the maximum of the first set, as well as their power. These form a triplet that
violates the conditions of construction.

So, we can choose any number of elements from each set, and we can choose
elements to be arbitrarily large. Specifically, let us
choose *x* from set *A* and *y* from set *B*, and let us
assume w.l.o.g. that *xy* is in *A*. Also, let us pick an element
*r* from *A* that is larger than *xy*.

In order not to create a triplet entirely in the same set, it can be deduced
that *r*^{x} and *r*^{xy} are both in *B*.
However, this leads to the triplet *a*=*r*^{x},
*b*=*y* and *c*=*r*^{xy}, all in set *B* - a
contradiction.

It is left to the reader to verify that all triplets constructed in this proof
are composed of three distinct elements.

#### Part 2:

Perhaps the most straightforward construction to solve this problem is to
admit primes into one set and composites into the other. Clearly, a prime can
never be represented as a non-trivial integer power.
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