The obstacle problem for the biharmonic operator
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 1, p. 151-184 
@article{ASNSP_1979_4_6_1_151_0,
     author = {Caffarelli, Luis and Friedman, Avner},
     title = {The obstacle problem for the biharmonic operator},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 6},
     number = {1},
     year = {1979},
     pages = {151-184},
     zbl = {0405.31007},
     mrnumber = {529478},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1979_4_6_1_151_0}
}

Caffarelli, Luis A.; Friedman, Avner. The obstacle problem for the biharmonic operator. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 1, pp. 151-184. http://www.numdam.org/item/ASNSP_1979_4_6_1_151_0/

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