no. 2

A class of second-order evolution equations with double characteristics
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 3 (1976) no. 2, pp. 187-229. 
@article{ASNSP_1976_4_3_2_187_0,
     author = {Gilioli, Antonio},
     title = {A class of second-order evolution equations with double characteristics},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {187--229},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 3},
     number = {2},
     year = {1976},
     zbl = {0327.35018},
     mrnumber = {437961},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1976_4_3_2_187_0/}
}
Gilioli, Antonio. A class of second-order evolution equations with double characteristics. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 3 (1976) no. 2, pp. 187-229. http://www.numdam.org/item/ASNSP_1976_4_3_2_187_0/

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[3] F. Trèves, Concatenations of second-order evolution equations applied to local solvability and hypoellipticity, Comm. Pure Appl. Math., 26 (1973), pp. 201-250. | MR 340804 | Zbl 0266.35060