1. R.P. Coleman, Theory of self-excited mechanical oscillations of hinged rotor blades, Technical Report NACA-WR-L-308, Langley Research Center, 1943, available from 〈ntrs.nasa.gov〉.
2. A.M. Feingold, Theory of mechanical oscillations of rotors with two hinged blades, Technical Report NACA-WR-L-312, Langley Research Center, 1943, available from 〈ntrs.nasa.gov〉.
3. R.P. Coleman, A.M. Feingold, Theory of ground vibrations of a two-blade helicopter rotor on anisotropic flexible supports, Technical Report NACA-TN-1184, Langley Research Center, 1947, available from 〈ntrs.nasa.gov〉.
4. Sur les équations différentielles linéaires à coefficients périodiques (on linear differential equations with periodic coefficients);Floquet;Annales scientifiques de l’École Normale Supérieure,1883
5. A Course of Modern Analysis;Whittaker,1927