Normalizers of the congruence subgroups of the Hecke group II

Mong-Lung Lang, Ser-Peow Tan
2000 Proceedings of the American Mathematical Society  
Let λ = 2 cos(π/5). Let (τ ) be an ideal of Z[λ] and let (τ 0 ) be the maximal ideal of Z[λ] such that (τ 2 0 ) ⊆ (τ ). Then N (G 0 (τ )) ≤ G 0 (τ 0 ). In particular, if τ is square free, then G 0 (τ ) is self-normalized in P SL 2 (R).
doi:10.1090/s0002-9939-00-05677-x fatcat:aj6xgijyprdebistvjvovjadpu