“…These methods are well described in the classic books (Hahn 2012, Lykov 1967 and have been used by many researchers for stationary and non-stationary temperature fields in multi-layer non-uniform structures. They are the orthogonal and quasi-orthogonal expansion technique (Tittle 1965, Bulavin 1965), Green's function approach (Siegel 1999, Haji-Sheikh 2002, Laplace transform method (Giere 1965, Lu 2013, Fourier transform method (Kayhani 2012), a finite integral transform (Singh 2011), a method of separation of variables and eigen function expansion (Norouzi 2016), for instance.…”