Baumslag-Solitar groups and residual nilpotence

For a Baumslag-Solitar group $G$ we calculate the intersection $\gamma_w(G)$ of all terms of the lower central sequence of $G$.Using this we are able to show that $[\gamma_w(G),G]=\gamma_w(G)$ thus answering a question of Bardakov and Neschadim. Finally we show that the quotient groups $\gamma_c(G)/\gamma_{c+1}(G)$ of the lower central series of $G$ are finite.