no. 2
Inner product-magnitude-preserving transformations in complex Hilbert spaces
Compositio Mathematica, Tome 21 (1969) no. 2, pp. 137-142 
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     author = {Beyer, W. A.},
     title = {Inner product-magnitude-preserving transformations in complex {Hilbert} spaces},
     journal = {Compositio Mathematica},
     pages = {137--142},
     year = {1969},
     publisher = {Wolters-Noordhoff Publishing},
     volume = {21},
     number = {2},
     mrnumber = {247514},
     zbl = {0175.43102},
     language = {en},
     url = {https://www.numdam.org/item/CM_1969__21_2_137_0/}
}
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Beyer, W. A. Inner product-magnitude-preserving transformations in complex Hilbert spaces. Compositio Mathematica, Tome 21 (1969) no. 2, pp. 137-142. https://www.numdam.org/item/CM_1969__21_2_137_0/

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