Geodesic
Teichmüller modular groups and arithmetical groups
Zapiski Nauchnykh Seminarov POMI, Investigations in topology. Part 6, Tome 167 (1988), pp. 95-110 
N. V. Ivanov. Teichmüller modular groups and arithmetical groups. Zapiski Nauchnykh Seminarov POMI, Investigations in topology. Part 6, Tome 167 (1988), pp. 95-110. http://geodesic.mathdoc.fr/item/ZNSL_1988_167_a5/
@article{ZNSL_1988_167_a5,
     author = {N. V. Ivanov},
     title = {Teichm\"uller modular groups and arithmetical groups},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {95--110},
     year = {1988},
     volume = {167},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1988_167_a5/}
}
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In this paper we give a negative answer to a question of W. Harvey about the arithmeticity of Teichmüller modular forms and we prove a number of results about the nonarithmeticity of subnormal subgroups of modular groups. In the Appendix we announce and discuss a theorem according to which the appropriately defined rank of Teichmüller modular groups is equal to 1.