1. Makin, A.S.,Differents. Uravn., 1999. vol. 35. no. 8. pp. 1058–1066.
2. Il’in, V.A.,Differents. Uravn, 1986, vol. 22, no. 12, pp. 2059–2071.
3. Shkalikov, A.A..Vestn. MGU. Mateniatika i Mekhanika, 1982, no. 6. pp. 12–21.
4. Naimark, M.A.,Lineinye diffemitsial’nye operatory (Linear Differential Operators), Moscow, 1969.
5. Ionian, N.I.,Differents. Uravn, 1977. vol. 13. no. 2. pp. 294–304.