1. The timing of the biological clocks;Winfree,1986
2. Theory of oscillators;Andronov,1989
3. The limit cycle of the van der Pol equation is not algebraic;Odani;J. Differen. Equat.,1995
4. The maximum number of limit cycles of the system ẏ=−x, ẋ=y−∑i=02aix2i+1 is two;Rychkov;Differen. Equat.,1975
5. On Liénard’s equation;Lins,1977