Moosa Gabeleh M., Moshokoa P., and Vetro C. “Cyclic (noncyclic) Phi-Condensing Operator and Its Application to a System of Differential Equations”. Nonlinear Analysis: Modelling and Control, Vol. 24, no. 6, Nov. 2019, pp. 985–1000, doi:10.15388/NA.2019.6.8.