Bulletin of the London Mathematical Society Advance Access originally published online on December 15, 2006
Bulletin of the London Mathematical Society 2007 39(1):53-62; doi:10.1112/blms/bdl001
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© 2006 London Mathematical Society
The structure of the normalisers of the congruence subgroups of the Hecke group G5
Department of Mathematics
National University of Singapore
Singapore 117543
matlml{at}math.nus.edu.sg
Received 8 July 2005. Revision received 23 March 2006.
Let
= 2cos (
/5) and let
[
]. Denote the normaliser of G0(
) of the Hecke group G5 in PSL2(
) by N(G0(
)). Then N(G0(
)) = G0(
/h), where h is the largest divisor of 4 such that h2 divides
. Further, N(G0(
))/G0(
) is either 1 (if h = 1),
2 x
2 (if h = 2) or
4 x
4 (if h = 4).