no. 1
Intersection numbers of curves on Hilbert modular surfaces
Séminaire Delange-Pisot-Poitou. Théorie des nombres, Tome 17 (1975-1976) no. 1, Exposé no. 22, 3 p. 
@article{SDPP_1975-1976__17_1_A20_0,
     author = {Hirzebruch, Friedrich},
     title = {Intersection numbers of curves on {Hilbert} modular surfaces},
     journal = {S\'eminaire Delange-Pisot-Poitou. Th\'eorie des nombres},
     note = {talk:22},
     pages = {1--3},
     publisher = {Secr\'etariat math\'ematique},
     volume = {17},
     number = {1},
     year = {1975-1976},
     zbl = {0352.14018},
     language = {en},
     url = {http://www.numdam.org/item/SDPP_1975-1976__17_1_A20_0/}
}
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Hirzebruch, Friedrich. Intersection numbers of curves on Hilbert modular surfaces. Séminaire Delange-Pisot-Poitou. Théorie des nombres, Tome 17 (1975-1976) no. 1, Exposé no. 22, 3 p. http://www.numdam.org/item/SDPP_1975-1976__17_1_A20_0/

[1] Zagier (D.). - Intersection numbers of curves on Hilbert modular surfaces and modular forms of Nebentypus, Inventiones math., t. 36, 1976, p. 57-113. | MR | Zbl