A group is said FC-group if for all $x\in G$ is true that the set $x^G$ is finite. Equivalently, $G$ is a FC-group if $|G:C_G(x)|$ is finite for all $x \in G$.

A group is said a central-by-finite if the center of $G$ has finite index $G$.

Is clear that if $G$ is a central-by-finite group then $G$ is a FC-group (in fact, $G$ is a BFC-group - look for the definition of BFC-grup in D. Robinson - Finiteness conditions and generalized soluble groups.

It is also true that: If G is a finitely generated FC-group, then G/Z(G) and Tor(G) are both finite.

So I do my question:

Let $G$ be a locally finite group. Suppose that $G$ is a FC-group. Then $G$ is central-by-finite?

Is true? If not, would like a counterexample.


1 Answer 1


A central product of countably infinitely many copies of $D_8$ (or more generally of any extraspecial group) is a counterexample. This group has the presentation

$$\langle x_i,y_i,z\ (i \in {\mathbb N}) \mid z^2=1,\ x_i^2=y_i^2=1,\ [x_i,y_i]=z\ (i \in {\mathbb N}),\ {\rm all\ other\ pairs\ of\ generators\ commute}\ \rangle.$$

Its centre is the finite subgroup $\langle z \rangle$. The conjugacy classes all have size $1$ or $2$, so it is a BFC-group.


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