Highlights on Poincare's research concerning semi-convergent series are given in this paper. The problem of the vanishing Hessian is analyzed with regard to both lines and non-linear differential equations in a special case studied by that author, showing that the relationships between periodic and formal solutions of the three-body problem should be investigated more closely. In this connection, further investigations should also take into account Barrar's results referred to the problem of normalization of the Hamiltonian in canonical systems.