Osuna, O., Rodríguez-Ceballos, J., Vargas-De-León, C., & Villaseñor-Aguilar, G. (2017). A note on the existence and construction of Dulac functions. Nonlinear Analysis: Modelling and Control, 22(4), 431-440. https://doi.org/10.15388/NA.2017.4.1