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, p. 567-589 
@article{ASNSP_1992_4_19_4_567_0,
     author = {Alessandrini, Giovanni and Magnanini, R.},
     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},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 19},
     number = {4},
     year = {1992},
     pages = {567-589},
     zbl = {0793.35021},
     mrnumber = {1205884},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1992_4_19_4_567_0}
}

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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