Kuznetsov N.V.; Kuznetsova O.A.; Leonov G.A. (2011). "Visualization of four normal size limit cycles in two-dimensional polynomial quadratic system". Differential Equations and Dynamical Systems. 21 (1–2): 29–33. doi:10.1007/s12591-012-0118-6. S2CID122896664.
Kuznetsov N.V.; Kuznetsova O.A.; Leonov G.A. (2011). "Visualization of four normal size limit cycles in two-dimensional polynomial quadratic system". Differential Equations and Dynamical Systems. 21 (1–2): 29–33. doi:10.1007/s12591-012-0118-6. S2CID122896664.