no. 2
On strictly singular operators
Compositio Mathematica, Tome 23 (1971) no. 2, pp. 169-183. 
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     author = {Van Dulst, D.},
     title = {On strictly singular operators},
     journal = {Compositio Mathematica},
     pages = {169--183},
     publisher = {Wolters-Noordhoff Publishing},
     volume = {23},
     number = {2},
     year = {1971},
     mrnumber = {288621},
     zbl = {0212.46402},
     language = {en},
     url = {http://archive.numdam.org/item/CM_1971__23_2_169_0/}
}
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Van Dulst, D. On strictly singular operators. Compositio Mathematica, Tome 23 (1971) no. 2, pp. 169-183. http://archive.numdam.org/item/CM_1971__23_2_169_0/

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S. Goldberg [3] Unbounded linear operators, theory and applications, McGraw-Hill, 1966. | MR | Zbl

T. Kato [4] Perturbation theory for nullity, deficiency and other quantities of linear operators, Journ. d'Anal. Math. 6 (1958), 273-322. | MR | Zbl