# Let $a, b$ and $n$ be natural numbers. Prove that if $a^n$ and $b^n$ are relatively prime, then $a$ and $b$ are relatively prime.

Let $$a, b$$ and $$n$$ be natural numbers. Prove that if $$a^n$$ and $$b^n$$ are relatively prime, then $$a$$ and $$b$$ are relatively prime.

I have been able to prove the above statement by contrapositive in the following way: If $$a$$ and $$b$$ are not relatively prime, then $$a^n$$ and $$b^n$$ are not relatively prime.

Suppose $$a$$ and $$b$$ are not relatively prime. Then $$\exists d \in \mathbb{N}$$ such that $$d \vert a \wedge d \vert b$$ where $$d>1$$.

Let $$n \in \mathbb{N}$$.

Since $$a^n = \overbrace{a\cdot a \dots \cdot a}^{n\text{-times}}$$ and $$b^n = \overbrace{b\cdot b \dots \cdot b}^{n\text{-times}}$$ then $$d \vert a^n \wedge d \vert b^n$$ which just implies that $$a^n$$ and $$b^n$$ are not relatively prime.

Could someone also show the direct proof?

• Your proof is correct and it is 'direct'. – Kabo Murphy Mar 21 at 10:06

Since $$\gcd(a^n,b^n)=1,\exists x,y\in\Bbb Z|a^nx+b^ny=1\implies a\cdot(a^{n-1}x)+b\cdot(b^{n-1}y)=1$$.
Thus, $$\exists p=a^{n-1}x,q=b^{n-1}y$$, both integers, such that $$ap+bq=1\implies\gcd(a,b)=1$$.
Hint: If a prime $$p$$ divides the product $$x=ab$$, then it divides a factor, i.e., $$p\mid a$$ or $$p\mid b$$.
$$\,\cal P(n) :=$$ set of prime factors of $$\,n.\,$$ By unique factorization, $$\,\cal \color{#c00}{P(a^k) = \cal P(a)}\,$$ for $$\,k\ge 1,\,$$ so
$$\gcd(a,b)\!=\!1\!\iff\! \cal P(a)\cap \cal P(b) = \emptyset\color{#c00}\!\iff\! P(a^m)\cap \cal P(b^n) =\emptyset \!\iff\! \gcd(a^m,b^n)\! =\! \rm1,\ \,m,n\ge 1$$