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2. Theory of Linear Operators in Hilbert Space;Akhiezer,1993
3. On the m-coefficient of Weyl for a differential equation with an indefinite weight function;Atkinson;Proc. London Math. Soc.,1974
4. On second order left definite boundary value problems;Bennewitz,1983
5. Expansions in Eigenfunctions of SelfAdjoint Operators;Berezanskiĭ,1968