Published online by Cambridge University Press: 08 June 2017
Let $w$ be a group-word. For a group
$G$, let
$G_{w}$ denote the set of all
$w$-values in
$G$ and let
$w(G)$ denote the verbal subgroup of
$G$ corresponding to
$w$. The group
$G$ is an
$FC(w)$-group if the set of conjugates
$x^{G_{w}}$ is finite for all
$x\in G$. It is known that if
$w$ is a concise word, then
$G$ is an
$FC(w)$-group if and only if
$w(G)$ is
$FC$-embedded in
$G$, that is, the conjugacy class
$x^{w(G)}$ is finite for all
$x\in G$. There are examples showing that this is no longer true if
$w$ is not concise. In the present paper, for an arbitrary word
$w$, we show that if
$G$ is an
$FC(w)$-group, then the commutator subgroup
$w(G)^{\prime }$ is
$FC$-embedded in
$G$. We also establish the analogous result for
$BFC(w)$-groups, that is, groups in which the sets
$x^{G_{w}}$ are boundedly finite.
The second author was supported by the ‘National Group for Algebraic and Geometric Structures and their Applications’ (GNSAGA – INdAM) and FAPDF-Brazil.