References
- Gilbarg , D. and Trudinger , N.S. 1977 . Elliptic Partial Differential Equation of Second Order , Berlin : Springer-Verlag .
- Mikhailov , V.P. 1976 . Differentsial’nye Uravneniya 1320 – 1329 .
- Showalter , R.E. 1977 . Hilbert Space Methods for Partial , Londan : Pitman .