1. D. Zwillinger, "Handbook of Differential Equations", 3rd edition Academic Press, 1997.
2. A.M. Wazwaz, The Variational iteration method for solving linear and nonlinear systems of PDEs, Comput. Math. Appl. 54: 895–902, 2007.
3. A.M. Wazwaz, Partial differential equations: Methods and applications, Balkema Publishers, The Netherlands, 2002.
4. L.N.M. Tawfiq, K.A. Jasim, and E.O. Abdulhmeed, Pollution of soils by heavy metals in East Baghdad in Iraq, International Journal of Innovative Science, Engineering & Technology, 2015.
5. G. Adomian, Solving frontier problems of physics: The decomposition method, Kluwer Academic Publishers, Boston and London, 1994.