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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     title = {On functions, whose lines of steepest descent bend proportionally to level lines},
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     year = {1983},
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     volume = {Ser. 4, 10},
     number = {4},
     mrnumber = {753157},
     zbl = {0542.35007},
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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. https://www.numdam.org/item/ASNSP_1983_4_10_4_587_0/

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