Посилання:Higher Order DiRerential Operators with Finite Number of d-Interactions in Multivariate Case / E.H. Eyvazov // Журнал математической физики, анализа, геометрии. — 2010. — Т. 6, № 3. — С. 266-276. — Бібліогр.: 9 назв. — англ.
A formula for calculating the defect number of a multi-center differential operator of higher order is obtained. The spectral properties of selfadjoint operators describing the finite number pointwise interactions are studied.