Spektralʹna︠i︡a teori︠i︡a differen︠t︡sialʹnykh operatorov : samosopr︠i︡azhennye differen︠t︡sialʹnye operatory - Ilʹin, V. A. (Nauka, Glav. red. fiz.-mat. lit-ry, 1991)