1. Yu. M. Berezanskii, Expansion in Eigenfucntions of Self-Adjoint Operators [in Russian], Naukova Dumka, Kiev (1965).
2. R. S. Ismagilov, ?Self-adjoint extensions of systems of commuting symmetric operators,? Dokl. Akad. Nauk SSSR,133, No. 3, 511?514 (1960).
3. R. S. Ismagilov ?Self-adjoint extensions of commuting symmetric operators,? Usp. Mat. Nauk,17, No. 1, 177?181 (1962).
4. Yu. E. Bokhonov, ?Commuting self-adjoint extensions of systems of Hermite operators,? Ukr. Mat. Zh.,40, No. 2, 149?153 (1988).