1. Su un Problema di Abel;Tonelli;Math. Ann.,1928
2. Hill's Equation;Magnus,1966
3. Attractive central forces may yield Liapunov instability;Zampieri,1986
4. A. Barone-Netto and M. O. Cesar, Nonconservative positional systems—Stability, to appear in Dynamics and Stability of Systems, Oxford Univ. Press.