1. An ‘improved’ van-der-Pol equation and some of its possible applications
2. KRYLOV , N. , and BOGOLIUBOV , N. , 1934 , Publication No. 8 of the Ukrainian Ac. Sci., Kiev (in Russian); Introduction to Non-linear Mechanics (a free translation by S. Lefschetz of excerpts from Russian Monographs) ( Princeton University Press, 1947 ).