The normal subgroup structure of the extended Hecke groupsKoruoğlu, Özden; Sahin, Recep; İkikardes, Sebahattin
doi: 10.1007/s00574-007-0035-4pmid: N/A
We consider the extended Hecke groups
$$
\ifmmode\expandafter\bar\else\expandafter\=\fi{H}{\left( \lambda \right)}
$$
generated by T(z) = −1/z, S(z) = −1/(z + λ) and R(z) = 1/z with λ ≥ 2. In this paper, firstly, we study the fundamental region of the extended Hecke groups
$$
\ifmmode\expandafter\bar\else\expandafter\=\fi{H}{\left( \lambda \right)}
$$
. Then, we determine the abstract group structure of the commutator subgroups
$$
{\ifmmode\expandafter\bar\else\expandafter\=\fi{H}}\ifmmode{'}\else$'$\fi{\left( \lambda \right)}
$$
, the even subgroup
$$
\ifmmode\expandafter\bar\else\expandafter\=\fi{H}_{e} {\left( \lambda \right)}
$$
, and the power subgroups
$$
\ifmmode\expandafter\bar\else\expandafter\=\fi{H}^{m} {\left( \lambda \right)}
$$
of the extended Hecke groups
$$
\ifmmode\expandafter\bar\else\expandafter\=\fi{H}{\left( \lambda \right)}
$$
. Also, finally, we give some relations between them.
Compact embedded rotation hypersurfaces of S n+1Li, Haizhong; Wei, Guoxin
doi: 10.1007/s00574-007-0037-2pmid: N/A
In this paper, we prove that
$$
S^{{n - 1}} {\left( {{\sqrt {\frac{{n - m}}
{n}} }} \right)} \times S^{1} {\left( {{\sqrt {\frac{m}
{n}} }} \right)}
$$
and round geodesic spheres are the only n-dimensional compact embedded rotation hypersurfaces with H
m
= 0 (1 ≤ m ≤ n − 1) in a unit sphere S
n+1(1). When m = 1, our result reduces to the result of T. Otsuki [O1], [O2], Brito and Leite [BL].