Geodesic
Regular and singular Hermitian operators
Matematičeskie zametki, Tome 8 (1970) no. 2, pp. 197-203.

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Let $A$ be a closed Hermitian operator, let $\mathfrak{H}'$ be the orthogonal complement of the domain of definition of $A$, and let $\mathfrak{R}_\lambda$ be the defect subspace. An operator $A$ is called regular if the orthogonal projection of $\mathfrak{H}'$ on $\mathfrak{R}_\lambda$ is closed. Criteria for regularity are established. 
@article{MZM_1970_8_2_a7,
     author = {Yu. L. Shmul'yan},
     title = {Regular and singular {Hermitian} operators},
     journal = {Matemati\v{c}eskie zametki},
     pages = {197--203},
     publisher = {mathdoc},
     volume = {8},
     number = {2},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1970_8_2_a7/}
}
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Yu. L. Shmul'yan. Regular and singular Hermitian operators. Matematičeskie zametki, Tome 8 (1970) no. 2, pp. 197-203. http://geodesic.mathdoc.fr/item/MZM_1970_8_2_a7/