Applied Mathematics
Vol.5 No.8(2014), Article ID:45904,8 pages DOI:10.4236/am.2014.58120
On Subsets of under the Action of Hecke Groups
Muhammad Aslam Malik1, Muhammad Asim Zafar2
1Department of Mathematics, University of the Punjab, Quaid-e-Azam Campus, Lahore, Pakistan
2Center for Undergraduate Studies, University of the Punjab, Quaid-e-Azam Campus, Lahore, Pakistan
Email: malikpu@yahoo.com, asimzafar@hotmail.com
Copyright © 2014 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/
Received 2 February 2014; revised 2 March 2014; accepted 9 March 2014
ABSTRACT
is the disjoint union of
for all
, where
is the set of all roots of primitive second degree equations
, with reduced discriminant
equal to k2m. We study the action of two Hecke groups—the full modular group
and the group of linear-fractional transformations
on
. In particular, we investigate the action of
on
for finding different orbits.
Keywords
Quadratic Irrationals, Hecke Groups, Legendre Symbol, G-Set
1. Introduction
In 1936, Erich Hecke (see [1] ) introduced the groups generated by two linear-fractional transformations
and
. Hecke showed that
is discrete if and only if
,
,
or
. Hecke group
is isomorphic to the free product of two finite cyclic group of order 2 and q, and it has a presentation
The first few of these groups are, the full modular group having special interest for mathematicians in many fields of Mathematics,
and
.
A non-empty set with an action of the group G on it, is said to be a G-set. We say that
is a transitive G-set if, for any
in
there exists a g in G such that
. Let
, where
and m is a square free positive integer. Then
is the set of all roots of primitive second degree equations, with reduced discriminant
equal to n and
is the disjoint union of for all k. If
and its conjugate
have opposite signs then
is called an ambiguous number [2] . The actual number of ambiguous numbers in
has been discussed in [3] as a function of n. The classification of the real quadratic irrational numbers
of
in the forms
modulo n has been given in [4] [5] . It has been shown in [6] that the action of the modular group
, where
and
, on the rational projective line
is transitive. An action of
, where
and
and its proper subgroups on
has been discussed in [7] [8] .
invariant under the action of modular group G but
is not invariant under the action of H. Thus it motivates us to establish a connection between the elements of the groups G and H and hence to deduce a common subgroup
of both groups such that each of
and
is invariant under H* and hence we find G-subsets of
and H-subsets of
or
according as
or
and
for all non-square n. Also the partition of has been discussed depending upon classes
modulo
.
2. Preliminaries
We quote from [5] [6] and [8] the following results for later reference. Also we tabulate the actions on of
and
, the generators of G and H respectively in Table 1.
Theorem 2.1 (see [5] ) Let,
. Then
and
are both G-subsets of
.
Theorem 2.2 (see [5] ) Let. Then
and
are both G-subsets of
.
Theorem 2.3 (see [6] ) If, then
and
are exactly two disjoint G-subsets of
depending upon classes
modulo 4.
Theorem 2.4 (see [6] ) If, then
and
Table 1. The action of elements of G and H on.
are both G-subsets of
.
Lemma 2.5 (see [8] ) Let. Then:
1) If then
if and only if
.
2) if and only if
.
Theorem 2.6 (see [8] ) The set, is invariant under the action of H.
Theorem 2.7 (see [8] ) For each non square positive integer,
is an H-subset of
.
3. Action of on
We start this section by defining a common subgroup of both groups and
, where
,
,
and
. For this, we need the following crucial results which show the relationships between the elements of G and H.
Lemma 3.1 Let and
be the generators of G and H respectively defined above. Then we have:
1) and
.
2) and
.
3) and
.
4) and
.
5) and
.
6) and
. In particular
and
.
Following corollary is an immediate consequence of Lemma 3.1.
Corollary 3.2 1) By Lemma 3.1, since and
so
is a common subgroup of G and H where
are the transformations defined by
and
.
2) As,
, so
is a proper subgroup of
.
3) and
.
Since for each integer n, either or
for each odd prime p. So in the following lemmawe classify the elements of
in terms of classes
with 0 modulo p or qr, qnr nature of a, b and c modulo p.
Lemma 3.3 Let be prime and
. Then
consists of classes
,
,
,
,
,
,
or
.
Proof. Let be any class of
. Then
leads us to exactly three cases. If
then exactly one of
is
and the other is qr or qnr of
as otherwise
and hence the class
is one of the forms
,
,
,
. If
then
and the class takes the form
or
. In third case if
then
so again
. This yields the class in the forms
or
. Hence the result.
Lemma 3.4 Let and let
be the class of
of
. Then:
1) If then
has the forms
,
,
,
,
,
,
,
,
,
,
or
only.
2) If then
has the forms
,
,
,
,
,
,
,
,
,
,
or
only.
Proof. Let be the class of
with
. As
so if
then according as
or
. Thus we have
,
if
and
,
if
. If
then
, so we get
,
,
,
,
,
,
,
,
or
only. This proof is now complete.
Lemma 3.5 Let and let
be the class of
of
. Then:
1) If then
has the forms
,
,
,
,
or
only.
2) If then
has the forms
,
,
,
,
or
only.
Proof. The proof is analogous to the proof of Lemma 3.4.
Note: If then
,
and
are three classes of
in modulo 2. If n is an odd then three classes of
are
,
and
modulo 2. These are the only classes of
if
. But if
then
is also a class of
and there are no further classes. These classes in modulo 2 of
do not give any useful information during the study of action of
on
except that if
then the set consisting of all elements of
of the form
is invariant under the action of the group G. Whereas the study of action of H* on
gives some useful information about these classes. The following crucial result determines the H*- subsets of
depending upon classes
modulo 2. It is interesting to observe that
splits into and
in modulo 2. Each of these two H*-subsets further splits into proper H*-subsets in modulo 4.
Lemma 3.6 and
are two distinct H*-subsets of
depending upon classes
modulo 2.
Theorem 3.7 and Remarks 3.8 are extension of Lemma 3.6 and discuss the action of H* on depending upon classes
modulo 4. Proofs of these follow directly by the equations
,
and classes
modulo 4 given in [6] .
Theorem 3.7 Let n be any non-square positive integer. Then splits into two proper H*- subsets
,
. Similarly
splits into two proper H*-subsets
and
.
Remark 3.8 1) Let. Then
and
are H*-subsets of
. In particular if
, then
and
are H*-subsets of
. Whereas if
, then
,
,
and
are H*-subsets of
. Specifically,
,
.
2) As we know that if and
are even, then
must be even as
. If
, then
and
.
3) If, then
or
is empty according as
or
. As we know that if n and c are even, then a must be even as
. However
,
are proper H*-subsets of
depending upon classes
modulo 4.
Lemma 3.9 Let n be any non-square positive integer. Then and
are distinct H*-subsets of an H-set
.
Proof. Follows by the equations and vice versa. Hence
is equivalent to
.
Clearly where
denotes the set of all ambigious numbers in
(see [8] ).
Remark 3.10 1) Each G-subset X of splits into two H*-subsets
and
and
.
2) Each H-subset Y of splits into two H*-subsets
and
.
3) Each H-subset Y of,
splits into two H*-subsets
and
.
4) Each H-subset Y of,
splits into two H*-subsets
and
.
Theorem 3.11 a) If A is an H*-subset of or
, then
is a G-subset of
.
b) If A is an H*-subset of, then
is an H-subset of
or
according as
or
.
c) If A is an H*-subset of, then
is an H-subset of
for all nonsquare n.
Proof. Proof of a) follows by the equation.
Proof of b) follows by the equations or
according as
or
.
Proof of c) follows by the equation.
Following examples illustrate the above results.
Example 3.12 1) Let. Then
but
. Also
but
. Similarly
whereas
. Also
,
. So
has exactly 4 orbits under the action of H whereas
splits into two G-orbits namely
,
.
2) splits into nine H-orbits. Also
and
. Whereas
splits into four
-orbits namely
,
and
. (see Figure 1)
Theorem 3.13 Let p be an odd prime factor of n. Then and
are two H*-subsets of
. In particular, these are the only H*- subsets of
depending upon classes
modulo p.
Figure 1. Orbit of and
.
Proof. Let be the class of
. In view of Lemma 3.3, either both of
are qrs or qnrs and the two equations,
fix b, c modulo p. If
then
or according as
or
. similarly for
. This shows that the sets
and
are H*-subsets of
depending upon classes modulo p.
The following corollary is an immediate consequence of Lemma 3.6 and Theorem 3.13.
Corollary 3.14 Let p be an odd prime and. Then
splits into four proper H*- subsets depending upon classes modulo 2p.
Proof. Since implies that
. This is equivalent to congruences
and
. By Theorem 3.13
,
are H*-subsets and then, by Lemma 3.6, each of
and
further splits into two H*-subsets
,
,
and
.
The next theorem is more interesting in a sense that whenever,
is itself an H*-set depending upon classes
modulo p.
Theorem 3.15 Let p be an odd prime and. Then
is itself an H*-set depending upon classes
modulo p.
Proof. follows from Lemmas 3.4, 3.5 and the equations and
given in Table 1.
Let us illustrate the above theorem in view of Theorem 3.4. If, then the set
is an H*-set.
That is, is itself an H*-set depending upon classes
modulo 3. Similarly for
.
Theorem 3.16 Let p be an odd prime and n is a quadratic residue (quadratic non-residue) of 2p. Then is the disjoint union of three H*-subsets
,
and
depending upon classes
modulo 2p.
Proof. Follows by Theorems 2.6, 2.7 and 3.15.
The following example justifies the above result.
Example 3.17 Since, then
splits into these three H*-subsets
,
,
.
The next theorem is a generalization of Theorem 3.13 to the case when n involves two distinct prime factors.
Theorem 3.20 Let and
be distinct odd primes factors of n. Then
,
,
and
are four H*-subsets of
. More precisely these are the only H*subsets of
depending upon classes
modulo
.
Proof. Let be any class of
with
. Then
implies that
(1)
This is equivalent to congruences and
. By Theorem 3.14, the congruence
gives two H*-subsets
and
of
. As
, again applying Theorem 3.13 on each of
and
we have four H*-subsets
,
,
and
of
.
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