no. 4
On functions, whose lines of steepest descent bend proportionally to level lines
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 10 (1983) no. 4, pp. 587-605. 
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     author = {Talenti, Giorgio},
     title = {On functions, whose lines of steepest descent bend proportionally to level lines},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {587--605},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 10},
     number = {4},
     year = {1983},
     zbl = {0542.35007},
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     url = {http://www.numdam.org/item/ASNSP_1983_4_10_4_587_0/}
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Talenti, Giorgio. On functions, whose lines of steepest descent bend proportionally to level lines. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 10 (1983) no. 4, pp. 587-605. http://www.numdam.org/item/ASNSP_1983_4_10_4_587_0/

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