14.41 (1999)

A group $G$ is said to be para-free if all factors $\gamma_i(G)/\gamma_{i+1}(G)$ of its lower central series are isomorphic to the corresponding lower central factors of some free group, and $\bigcap_{i=1}^\infty \gamma_i(G) = 1$. Is an arbitrary para-free group representable by matrices over a commutative-associative ring with 1?