Ciałkowski, M.J. and Frąckowiak A. (2000), Heat functions and their application for solving heat transfer and mechanical problems, Poznan´ University of Technology Publishers, (in Polish). Grysa, K. (2010), Trefftz functions and their application in solving the inverse problems, Kielce University of Technology Publishers (in Polish), Grysa, K., Maciag, A. and Pawinska A. (2012), Solving nonlinear direct and inverse problems of stationary heat transfer by using Trefftz functions, Int. J. Heat and Mass Transfer 55, 23-24, pp. 7336–7340, Grysa, K. and Maciejewska, B. (2013), Trefftz functions for non-stationary problems, Journal of Theoretical and Applied Mechanics, 51, 2, pp.251-264 He, J.-H. (2000), A coupling method of homotopy technique and perturbation technique for nonlinear problems. Internat. J. Non-linear Mech. 35(1) pp. 37–43. He J,-H. (1999), Homotopy perturbation technique. Comput. Math. Appl. Mech. Eng. 178(3-4) pp. 257–262. Jin, L. (2008), Homotopy Perturbation Method for Solving Partial Differential Equations with Variable Coefficients, Int. J. Contemp. Math. Sciences, 28(3) pp. 1395 – 1407. Quarnia El, H. (2009), Application of homotopy perturbation method to non-homogeneous parabolic partial and non linear differential equations. World J. of Modelling and Simulation 5(3) pp. 225-231. Trefftz E. (1926), Ein Gegenstuek zum Ritz'schen Verfahren, Proceedings 2nd International Congress of Applied Mechanics (Zurich), pp. 131–137.