This article is within the scope of WikiProject Mathematics, a collaborative effort to improve the coverage of mathematics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.MathematicsWikipedia:WikiProject MathematicsTemplate:WikiProject Mathematicsmathematics
I've tried to add a bit to the article -- it wasn't so clear to me what the relation to congruence subgroups was meant to be, or what was meant to represent -- unfortunately the cited reference is several hundred pages! So I've left this alone for now, but it should be clarified.