@article{ASNSP_2003_5_2_1_21_0,
     author = {Costin, Ovidiu and Costin, Rodica D.},
     title = {Failure of analytic hypoellipticity in a class of differential operators},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {21--45},
     publisher = {Scuola normale superiore},
     volume = {Ser. 5, 2},
     number = {1},
     year = {2003},
     mrnumber = {1990973},
     zbl = {1150.35018},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_2003_5_2_1_21_0/}
}