no. 4
The index of isolated critical points and solutions of elliptic equations in the plane
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 19 (1992) no. 4, pp. 567-589. 
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     title = {The index of isolated critical points and solutions of elliptic equations in the plane},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
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Alessandrini, G.; Magnanini, R. The index of isolated critical points and solutions of elliptic equations in the plane. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 19 (1992) no. 4, pp. 567-589. http://www.numdam.org/item/ASNSP_1992_4_19_4_567_0/

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