Suppose $G=\left\langle x, y, t\mid x^7=y^7=t^3=1, txt^{-1}=x^2, tyt^{-1}=y\right\rangle$. Show that $y\in Z(G)$.

Question

The Problem: Suppose $G=\left\langle x, y, t\mid x^7=y^7=t^3=1, txt^{-1}=x^2, tyt^{-1}=y\right\rangle$. Show that $y\in Z(G)$.

My Attempt: Clearly $y$ commutes with $t$, so $y$ commutes with $t^2$ as well. Thus to show $y\in Z(G)$, it suffices to show that $y$ commutes with $x$. But I struggle to prove that $xy=yx$. It seems a rather silly place to get stuck-any HINT would be greatly appreciated.

Answer 1

That's not correct. For example. Let $$ x=(1,2,3,4,5,6,7),\ y=(5,8,9,10,11,12,13),\ t=(1,3,4)(2,7,6). $$ It is easy to check that $x^7=y^7=t^3=1$, $txt^{-1}=x^2$, and $tyt^{-1}=y$. However, $xy\neq yx$.

Answer 2

Further to @kabenyuk's excellent answer, one way to see that this is false is to use GAP:

gap> F:=FreeGroup(3);
<free group on the generators [ f1, f2, f3 ]>
gap> x:=F.1; y:=F.2; t:=F.3;
f1
f2
f3
gap> rels:=[x^7, y^7, t^3, t*x*t^(-1)*x^(-2), t*y*t^(-1)*y^(-1)];
[ f1^7, f2^7, f3^3, f3*f1*f3^-1*f1^-2, f3*f2*f3^-1*f2^-1 ]
gap> G:=F/rels;
<fp group on the generators [ f1, f2, f3 ]>
gap> G.2*G.3=G.3*G.2;  #This is a sanity check: It must return true.
true
gap> G.1*G.2=G.2*G.1;
false