@article{Bautin,
author={Bautin},
year={1952},
title={On the number of limit cycles appearing on varying the coefficients from a focus or centre type of equilibrium state},
journal={Mat Sb (N.S.)},
volume={30},
pages={181-196},
url={www.scopus.com},
}
@book{Perko2000,
author = {Perko},
isbn = {0387951164},
title = {{Equations and Dynamical Systems}},
year = {2000}
}