Published online by Cambridge University Press: 07 May 2015
We study the free Lie ring of rank $2$ in the variety of all centre-by-nilpotent-by-abelian Lie rings of derived length
$3$. This is the quotient
$L/([\unicode[STIX]{x1D6FE}_{c}(L^{\prime }),L]+L^{\prime \prime \prime })$ with
$c\geqslant 2$ where
$L$ is the free Lie ring of rank
$2$,
$\unicode[STIX]{x1D6FE}_{c}(L^{\prime })$ is the
$c$th term of the lower central series of the derived ideal
$L^{\prime }$ of
$L$, and
$L^{\prime \prime \prime }$ is the third term of the derived series of
$L$. We show that the quotient
$\unicode[STIX]{x1D6FE}_{c}(L^{\prime })+L^{\prime \prime \prime }/[\unicode[STIX]{x1D6FE}_{c}(L^{\prime }),L]+L^{\prime \prime \prime }$ is a direct sum of a free abelian group and a torsion group of exponent
$c$. We exhibit an explicit generating set for the torsion subgroup.