El-Kabeir, S. M. M. M. M., et al. “Natural Convection from a Permeable Sphere Embedded in a Variable Porosity Porous Medium Due to Thermal Dispersion”. Nonlinear Analysis: Modelling and Control, vol. 12, no. 3, July 2007, pp. 345-57, https://doi.org/10.15388/NA.2007.12.3.14693.