1. Akhieser, N.I., Glazman, I.M.: Theory of linear operators in Hilbert space I and II. Ungar Publishers, New York (1963)
2. Behrndt, H., Hassi, S., de Snoo, H.: Boundary value problems, Weyl functions and differential operators, monographs in mathematics. Birkhäuser Publishers, Basel (2020)
3. Chisholm, R.S., Everitt, W.N.: On bounded integral operators in the space of integrable-square functions. Proc. R. Soc. Edinb. Sect. A Math. 69, 199–204 (1971)
4. Elliott, K.: The sixth-order Krall differential expression and self-adjoint operators. Ph.D. thesis, Baylor University, Waco (2019)
5. Everitt, W.N., Littlejohn, L.L.: Differential operators and the Legendre type polynomials. Differ. Integr. Equ. 1(1), 97–116 (1988)