1. Nonlinear Dynamics and Chaos;Strogatz,1994
2. Oscillations in Planar Dynamic Systems;Mickens,1996
3. Period Motions;Farkas,1994
4. Nonlinear Ordinary Differential Equations;Jordan,1987
5. R.E. Mickens, J. Ernest Wilkins Jr., Unpublished calculations, Clark Atlanta University, Atlanta, GA, June 2002.