# A group such that $a^m b^m = b^m a^m$ and $a^n b^n = b^n a^n$ ($m$, $n$ coprime) is abelian?

Let $(G,.)$ be a group and $m,n \in\mathbb Z$ such that $\gcd(m,n)=1$. Assume that $$\forall a,b \in G, \,a^mb^m=b^ma^m,$$ $$\forall a,b \in G, \, a^nb^n=b^na^n.$$

Then how prove $G$ is an abelian group?

Some context: Some of these commutation relations often imply that $G$ is abelian, for example if $(ab)^i = a^i b^i$ for three consecutive integers $i$ then $G$ is abelian, or if $g^2 = e$ for all $g$ then $G$ is abelian. This looks like another example of this phenomenon, but the same techniques do not apply.

• from which text did you get this problem ? – Fawzy Hegab Mar 10 '13 at 19:32
• also , is m,n are fixed or you mean , for all m,n in Z such that gcd(m,n)=1 .... ??? – Fawzy Hegab Mar 10 '13 at 19:39
• @ MrWhymy:m,n are fixed integer number – M.H Mar 10 '13 at 19:46
• it's nice problem , and the solution which is providen in the answer below is great :) – Fawzy Hegab Mar 10 '13 at 19:53

Let $M \subset G$ be the subgroup generated by all $m$-th powers and let $N \subset G$ be the subgroup generated by all $n$-th powers. These subgroups are clearly abelian normal subgroups. Since $m$ and $n$ are coprime, $G = MN$, and hence $M \cap N$ is contained in the center $Z(G)$ of $G$. To prove that $G$ is abelian it suffices to show that $M$ and $N$ commute, that is $[M,N]=1$. Note that $[M,N] \subset M \cap N$ (since $M$ and $N$ are normal subgroups). Let $a \in M$ and $b \in N$. Then $[a, b] = a^{−1}b^{−1}ab \in M \cap N$. Hence $[a, b] = z$ with $z \in Z(G)$. Hence $b^{−1}ab = za$, whence $b^{−1}a^nb=z^na^n$. Since $a^n \in N$ it commutes with $b$, so $z^n=1$. Similarly $z^m=1$. Since $m$ and $n$ are relatively prime, we conclude $z=1$.

• Dear Nicky! Let me ask sime questions: 1) Why $G=MN$? 2) Why $M\cap N$ is contained in the center $Z(G)$? – ZFR May 1 '18 at 14:21
• (1) $gcd(m,n)=1$ whence you can find integers $k$ and $l$ with $1=km+ln$. So if $g \in G$ then $g=g^1=(g^k)^m \cdot (g^l)^n \in MN$. (2) Because $G=MN$ and $M$ and $N$ are abelian, any element from $M \cap N$ commutes with any element from $G$. Hope it is clear now. – Nicky Hekster May 1 '18 at 15:33
• Dear Nicky! Thanks for answer! I got your solution and your approach is very simple and nice! +1 – ZFR May 1 '18 at 19:53
• Thank you very much! – Nicky Hekster May 1 '18 at 20:32

Write $mk+nl=1$ for some $k,l\in\mathbb Z$.

We have $$ab=a^{mk+nl}b^{mk+nl}=a^{mk}(a^{nl}b^{mk})b^{nl}\stackrel{(*)}=a^{mk}(b^{mk}a^{nl})b^{nl}=(a^{mk}b^{mk})(a^{nl}b^{nl})=(b^{mk}a^{mk})(b^{nl}a^{nl})=b^{mk}(a^{mk}b^{nl})a^{nl}\stackrel{(**)}=b^{mk}(b^{nl}a^{mk})a^{nl}=(b^{mk}b^{nl})(a^{mk}a^{nl})=b^{mk+nl}a^{mk+nl}=ba,$$ where we used $$a^{nl}b^{mk}=b^{mk}a^{nl}\qquad(*)$$ and $$a^{mk}b^{nl}=b^{nl}a^{mk}\qquad(**)$$

Let's prove $(*)$ and $(**)$:

$$(a^mb^n)^{mk}=a^m(b^na^m)^{mk-1}b^n=a^m(b^na^m)^{mk}a^{-m}\Rightarrow(a^mb^n)^{mk}a^m=a^m(b^na^m)^{mk}\Rightarrow a^m(a^mb^n)^{mk}=a^m(b^na^m)^{mk},$$ hence $$(a^mb^n)^{mk}=(b^na^m)^{mk}.\qquad(\sharp)$$

Similarly we get $$(a^mb^n)^{nl}=(b^na^m)^{nl}.\qquad(\sharp\sharp)$$

Multiplying $(\sharp)$ and $(\sharp\sharp)$ we obtain $a^mb^n=b^na^m$.

Proceeding in the same way we get $a^nb^m=b^ma^n$

An easy induction shows now $(*)$ and $(**)$.

As $\gcd(m, n) = 1$, by Bezóut's identity there are $u$, $v$ such that $u m + v n = 1$.

Take $a, b \in G$ arbitrary. By the conditions, as $a^u \in G$ for all $u \in \mathbb{Z}$, and as the conditions allow to conmute certain powers: \begin{align*} a^{u m} b^{u m} &= b^{u m} a^{u m} \\ (a^{u m} b^{u m})^{v n} &= a^{u m + v n} b^{u m + v n} \\ &= a b \\ (b^{u m} a^{u m})^{v n} &= b^{u m + v n} a^{u m + v n} \\ &= b a \end{align*} This because e.g. $(a^{u m} b^{u m})^2 = a^{u m} b^{u m} a^{u m} b^{u m} = a^{u m} a^{u m} b^{u m} b^{u m} = a^{2 u m} b^{2 u m}$ since the middle pair conmutes by the conditions. Now use induction.

(Perhaps the switching around can be expressed in a more compact way).

Nice problem!

• Great Solution :) thanx :) – Fawzy Hegab Mar 10 '13 at 19:52
• @vonbrand:why we have $(b^{u m} a^{u m})^{v n} = b^{u m + v n} a^{u m + v n}$? – M.H Mar 10 '13 at 20:30
• @MaisamHedyelloo , we have a in G , so $a^u$ is in G , choose u to be the same u in the relation $mu+vn=1$ , apply the hyposetes to $a_1$ = $a^u$ and use induction as it's mentioned in the answer in the speicial case for 2 . – Fawzy Hegab Mar 10 '13 at 20:45
• How do you get $(a^{u m} b^{u m})^{v n} = a^{u m + v n} b^{u m + v n}$? Doesn't $(a^{u m} b^{u m})^{v n} = a^{u m v n} b^{u m v n}$? – Code-Guru Mar 10 '13 at 22:11
• @MrWhy Huh? What's $a_1$? – Code-Guru Mar 10 '13 at 22:15