Convexity of balls and fixed-point theorems for mappings with nonexpansive square
Compositio Mathematica, Tome 22 (1970) no. 3, pp. 269-274. 
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     author = {Goebel, K.},
     title = {Convexity of balls and fixed-point theorems for mappings with nonexpansive square},
     journal = {Compositio Mathematica},
     pages = {269--274},
     publisher = {Wolters-Noordhoff Publishing},
     volume = {22},
     number = {3},
     year = {1970},
     mrnumber = {273477},
     zbl = {0202.12802},
     language = {en},
     url = {http://www.numdam.org/item/CM_1970__22_3_269_0/}
}
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Goebel, K. Convexity of balls and fixed-point theorems for mappings with nonexpansive square. Compositio Mathematica, Tome 22 (1970) no. 3, pp. 269-274. http://www.numdam.org/item/CM_1970__22_3_269_0/

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