1. Expansions in eigenfunctions of selfadjoint operators;Berezanskii;Amer. Math. Soc. Transl.,1968
2. Comparison theorems for elliptic differential equations;Clark,1965
3. Direct Methods of Qualitative Spectral Analysis of Singular Differential Equations;Glazman,1965
4. The least cluster point of the spectrum of boundary value problems;Hartman;Amer. J. Math.,1948
5. Some oscillation properties of selfadjoint elliptic equations;Headley,1970