Addendum to: Symbolic dynamics for the $N$-centre problem at negative   energies
  This paper aims at completing and clarifying a delicate step in the proof of Theorem 5.3 of our paper \cite{ST}, where it was used the differentiability of a function $F$, which a priori can appear not necessarily differentiable. 
Nicola Soave and Susanna Terracini
math.DS