1. G. Boole,A Treatise on Differential Equations, Chelsea Publishing Company, London, 1859.
2. Why the ant trails look so straight and nice
3. H. T. Davis,Introduction to Nonlinear Differential and Integral Equations, Dover, NY, 1962.
4. R. L. Graham, D. E. Knuth, and O. Patashnik,Concrete Mathematics, Addison-Wesley, Reading, MA, 1991.