The Hecke Group <math xmlns="http://www.w3.org/1998/Math/MathML" id="M1"> <mi>H</mi> <mfenced open="(" close=")" separators="|"> <mrow> <msub> <mrow> <mi>λ</mi> </mrow> <mrow> <mn>4</mn> </mrow> </msub> </mrow> </mfenced> </math> Acting on Imaginary Quadratic Number Fields
Journal of mathematics