no. 1
Kuttner's theorem for operators
Compositio Mathematica, Tome 29 (1974) no. 1, pp. 35-41 
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     author = {Maddox, I. J.},
     title = {Kuttner's theorem for operators},
     journal = {Compositio Mathematica},
     pages = {35--41},
     year = {1974},
     publisher = {Noordhoff International Publishing},
     volume = {29},
     number = {1},
     mrnumber = {361524},
     zbl = {0296.46016},
     language = {en},
     url = {https://www.numdam.org/item/CM_1974__29_1_35_0/}
}
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Maddox, I. J. Kuttner's theorem for operators. Compositio Mathematica, Tome 29 (1974) no. 1, pp. 35-41. https://www.numdam.org/item/CM_1974__29_1_35_0/

[1] B. Kuttner: Note on strong summability. J. London Math. Soc., 21 (1946) 118-122. | Zbl | MR

[2] G.G. Lorentz and M.S. Macphail: Unbounded operators and a theorem of A. Robinson. Trans. Royal Soc. of Canada, XLVI, Series III (1952) 33-37. | Zbl | MR

[3] I.J. Maddox: On Kuttner's theorem. J. London Math. So., 43 (1968) 285-290. | Zbl | MR

[4] I.J. Maddox: Elements of functional analysis (Cambridge University Press, 1970). | Zbl | MR

[5] A. Robinson: On functional transformations and summability. Proc. London Math. Soc., (2), 52 (1950) 132-160. | Zbl | MR