Geodesic
On multiple completeness of eigen functions of differential operator bunch generated by general boundary conditions
Proceedings of the Yerevan State University. Physical and mathematical sciences, no. 3 (1987), pp. 3-8.

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For (I), (II) spectral problem with general boundary conditions (when the $\Omega$ region is single circle) the multiple completeness of the eigenfunctions in Sobolev’s space $^0 W^1_2 (\Omega)$ has been proved. 
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G. V. Virabyan; G. A. Sargsian. On multiple completeness of eigen functions of differential operator bunch generated by general boundary conditions. Proceedings of the Yerevan State University. Physical and mathematical sciences, no. 3 (1987), pp. 3-8. http://geodesic.mathdoc.fr/item/UZERU_1987_3_a0/

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