We determine all modular curves X(N) (with N ≥ 7) that are hyperelliptic or bielliptic. We also give a proof that the automorphism group of X(N) is PSL₂(ℤ/Nℤ), whence it coincides with the normalizer of Γ(N) in PSL₂(ℝ) modulo ±Γ(N).
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-aa161-3-6, author = {Francesc Bars and Aristides Kontogeorgis and Xavier Xarles}, title = {Bielliptic and hyperelliptic modular curves X(N) and the group Aut(X(N))}, journal = {Acta Arithmetica}, volume = {161}, year = {2013}, pages = {283-299}, zbl = {1304.11052}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-aa161-3-6} }
Francesc Bars; Aristides Kontogeorgis; Xavier Xarles. Bielliptic and hyperelliptic modular curves X(N) and the group Aut(X(N)). Acta Arithmetica, Tome 161 (2013) pp. 283-299. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-aa161-3-6/