# If $G$ is amenable, when $G\times G$ is amenable ?

I am not specialist on Topological Group Theory, I apologize if this is a trivial question.

Question. If $G_1=G_2$ are amenable topological groups what additional hypothesis we have to consider on the group, in order to prove that $G_1\times G_2$ is amenable ?

Following Leinster, in this question Why are abelian groups amenable?,
"The direct product of two amenable groups is amenable. This isn't exactly trivial, but the measure on the product is at least constructed canonically from the two given measures."
So discreteness of the $G_1$ and $G_2$ are enough to prove that $G_1\times G_2$ is amenable and also we do not need to suppose that $G_1=G_2$.

Looking for a proof, in more general cases, I found the following statement:
"... direct product $G_1\times G_2$ of two countable amenable groups can not be amenable."
in the paper, On Subadditive Processes on Direct Product of Countable Amenable Groups by Seyit Temir - Publications De l'Institute Mathématique (2002), 119-122.

Since the author did not mention if the example needs two different groups and I have no access to the paper containing this information, I decided to post this question.

• I'm confused-- extensions of amenable groups are amenable-- that is, if N is a normal subgroup of G and both N and $G/N$ are amenable, then so is G. And trivially $G$ is a normal subgroup of $G\times G$, with the quotient being $G$... – Matthew Daws Mar 18 '11 at 10:43
• Hi Matthew, of course your observation also works for different factors $G_1$ and $G_2$, now I would like to understand what is Temir's point. – Leandro Mar 18 '11 at 15:29
• I have just had a look at the paper emis.de/journals/PIMB/086/13.html and I am confused, verging on appalled. I will perhaps have a closer look later, but my current feeling is that the author has misunderstood something. The list of references is a bit suspicious too. – Yemon Choi Mar 18 '11 at 17:28
• MathReview for that Temir paper: ams.org/mathscinet-getitem?mr=1997617 There is no comment made on the staggering claim that it is unknown if the product of two countable amenable groups is amenable, and the MR in some ways appalls me even more than the original paper – Yemon Choi Mar 18 '11 at 23:36
• Maybe someone confused the true claim that a countably infinite direct product of finite amenable groups need not be amenable with the false claim that a finite direct product of countably infinite amenable groups need not be amenable? – Alexander Pruss Feb 1 '18 at 16:49

Say that a topological group $G$ is amenable if every continuous affine action of $G$ on a non-empty compact convex subset of a locally convex topological space, has a fixed point. With this definition, it is completely standard to prove that, if $N$ is a closed normal subgroup of $G$ such that $N$ and $G/N$ are amenable, then $G$ is amenable.