1. M.G. Krein, Dokl. Akad. Nauk SSSR 69 (2), 125 (1949).
2. N.I. Akhiezer, The Classical Moment Problem and Some Related Questions in Analysis (Fizmatgiz, Moscow, 1961) [in Russian].
3. Yu.M. Berezanskii, Eigenfunction Expansions of Self-Adjoint Operators (Naukova Dumka, Kiev, 1965) [in Russian].
4. A. G. Kostyuchenko and K. A. Mirzoev, Mat. Zametki 63 (5), 709 (1998) [Math. Notes 63 (5), 624 (1998)].
5. A. G. Kostyuchenko and K.A. Mirzoev, Funktsional. Anal. Prilozhen. 35 (4), 32 (2001) [Functional Anal. Appl. 35 (4), 265 (2001)].