no. 1
Star shaped coincidence sets in the obstacle problem
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 11 (1984) no. 1, pp. 123-128 
@article{ASNSP_1984_4_11_1_123_0,
     author = {Sakaguchi, Shigeru},
     title = {Star shaped coincidence sets in the obstacle problem},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {123--128},
     year = {1984},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 11},
     number = {1},
     mrnumber = {752582},
     zbl = {0539.49004},
     language = {en},
     url = {https://www.numdam.org/item/ASNSP_1984_4_11_1_123_0/}
}
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Sakaguchi, Shigeru. Star shaped coincidence sets in the obstacle problem. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 11 (1984) no. 1, pp. 123-128. https://www.numdam.org/item/ASNSP_1984_4_11_1_123_0/

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