@inproceedings{erik1999spectral,
title={Spectral theory of Laplacians for Hecke groups with primitive character},
author={Erik Balslev, and Alexei Venkov},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203338004991607},
booktitle={Acta Mathematica},
volume={186},
number={2},
pages={155-217},
year={1999},
}
Erik Balslev, and Alexei Venkov. Spectral theory of Laplacians for Hecke groups with primitive character. 1999. Vol. 186. In Acta Mathematica. pp.155-217. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203338004991607.