On the form of functions which preserve regressive isols
Compositio Mathematica, Tome 26 (1973) no. 3, pp. 283-302. 
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Ellentuck, Erik. On the form of functions which preserve regressive isols. Compositio Mathematica, Tome 26 (1973) no. 3, pp. 283-302. http://archive.numdam.org/item/CM_1973__26_3_283_0/

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