[1] S. Wolfram, “The Mathematica book,” 5th Ed., Cambridge University Publications, 2003.
[2] E.g. J. Marion, “Classical dynamics of particles and systems,” 4th Ed., Harcourt College Publishers, 1995.
[3] G. Duffing, “Erzwungene schwingungen bei veranderlicher eigen-frequenz,” F. Vieweg und Sohn, Braunschweig, 1918.
[4] S. Strogatz, “Nonlinear dynamics and chaos,” Perseus Publishing, 1994.
[5] http://wow.scalped.orgy/article/Duffing_oscillator.
[6] R. H. Ennis and G. C. McGuire, “Nonlinear physics with Mathematica for scientists and engineers,” Published by Birdhouse, Hard Spring, Peg 605, 2001.
[7] E.g. J. D. Jackson, “Classical electrodynamics,” 3rd Ed., Wiley, 2005.