1. F. S. Rofe-Beketov, ?Self-adjoint extensions of differential operators in a space of vector functions,? Dokl. Akad. Nauk SSSR,184, No. 5, 1034?1037 (1969).
2. R. Arens, ?Operational calculus of linear relations,? Pac. J. Math.,11, No. 1, 9?23 (1961).
3. E. A. Coddington, ?Extension theory of formally normal and symmetric subspaces,? Mem. Am. Math. Soc.,134, 1?80 (1973).
4. A. Dijksma and H. S. V. de Snoo, ?Self-adjoint extensions of symmetric subspaces,? Pac. J. Math.,54, No. 1, 71?100 (1974).
5. A. Pleijel, ?A survey of spectral theory for pairs of ordinary differential operators,? in: Lect. Notes Math., Vol. 448, Springer-Verlag, New York (1975), pp. 256?272.