no. 4
An inversion of the obstacle problem and its explicit solution
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 4, pp. 561-571 
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     author = {Lewy, Hans},
     title = {An inversion of the obstacle problem and its explicit solution},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {561--571},
     year = {1979},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 6},
     number = {4},
     mrnumber = {563334},
     zbl = {0428.35080},
     language = {en},
     url = {https://www.numdam.org/item/ASNSP_1979_4_6_4_561_0/}
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Lewy, Hans. An inversion of the obstacle problem and its explicit solution. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 4, pp. 561-571. https://www.numdam.org/item/ASNSP_1979_4_6_4_561_0/

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[4] D.G. Schaeffer, A stability theorem for the obstacle problem, Advances in Math., 17 (1975), pp. 34-47. | Zbl | MR