1. V. I. Gorbachuk and M. L. Gorbachuk, ?Some problems in the spectral theory of elliptic-type differential equations in a space of vector-functions on a finite interval,? Ukr. Mat. Zh.,28, No. 1, 12?26 (1976).
2. V. I. Gorbachuk and M. L. Gorbachuk, Boundary Problems for Differential-Operator Equations [in Russian], Naukova Dumka, Kiev (1984).
3. Yu. M. Berezanskii, Expansions in Eigenfunctions of Selfadjoint Operators. Translations of Math. Monographs, Vol. 17, Am. Math. Soc., Providence, Rhode Island (1968).
4. M. Sh. Birman and M. Z. Solomyak, Spectral Theory of Self-Adjoint Operators in Hubert Space [in Russian], Leningrad State Univ. (1970).
5. V. I. Gorbachuk and M. L. Gorbachuk, ?On some classes of boundary problems for a Sturm-Liouville equation with operator potential,? Ukr. Mat. Zh.,24, No. 6, 726?734 (1972).