# Non-commutative finite monoids of a given order n ≥ 4 ```DOI: 10.2478/auom-2014-0028
An. Şt. Univ. Ovidius Constanţa
Vol. 22(2),2014, 29–35
Non-commutative finite monoids
of a given order n ≥ 4
B. Ahmadi, C.M. Campbell and H. Doostie
Abstract
α
1 α2
k
For a given integer n = pα
1 p2 . . . pk , (k ≥ 2), we give here a
class of finitely presented finite monoids of order n. Indeed the monoids
M on(π), where
α
p i
π = ha1 , a2 , . . . , ak |ai i = ai , (i = 1, 2, . . . , k), ai ai+1 = ai , (i =
1, 2, . . . , k − 1)i.
As a result of this study we are able to classify a wide family of the kgenerated p-monoids (finite monoids of order a power of a prime p). An
interesting difference between the center of finite p-groups and the center
of finite p-monoids has been achieved as well. All of these monoids are
regular and non-commutative.
1. Introduction
The study of finite monoids is of interest because of its applications in several
branches of science, for instance, its uses and advantages in mathematics,
computer science and finite machines are well-known. So identifying a finite
monoid of a given positive integer n could be significant. In this paper we
present a class of finite monoids for every integer n.
First of all we give a short history on the finitely presented semigroups and
monoids. Let A be an alphabet. We denote by A+ the free semigroup on A
Key Words: Semigroup and monoid presentation, Regular semigroup, Inverse
semigroup.
2010 Mathematics Subject Classification: 20M05, 20M17, 20M18.
Revised: February 2013
Accepted: May 2013
29
Non-commutative finite monoids of a given order n ≥ 4
30
consisting of all non-empty words over A, and by A∗ the free monoid A+ ∪{1},
where 1 denotes the empty word. A semigroup (or monoid) presentation is
an ordered pair hA|Ri, where R ⊆ A+ &times; A+ (or R ⊆ A∗ &times; A∗ ). A semigroup
(or monoid) presentation S is said to be defined by the semigroup (or monoid)
presentation hA|Ri if S ∼
= A∗ /ρ), where ρ is the congruence
= A+ /ρ (or S ∼
+
∗
on A (or A ) generated by R. Note that the semigroup presentations for a
semigroup S are precisely monoid presentations (without the trivial relation
1=1) for the monoid S 1 obtained from S by adjoining an identity, whether or
Here, our notation is standard and we follow [5, 11, 12]. one may consult  for more information on the presentation of groups. To distinguish
between the semigroup, the monoid and the group defined by a presentation
π = hA|Ri, we shall denote them by Sg(π), M on(π) and Gp(π), respectively.
On comparing the semigroup, monoids and groups defined by a presentation the articles [1, 2, 3, 9] studied certain interesting classes of such algebraic
structure. The references [6, 7] study two special and outstanding classes of
semigroups. For the recently obtained results on the study of subsemigroups
and the efficiency of semigroups one may see the interesting results [4, 8].
Considering the presentation
p
αi
π = ha1 , a2 , . . . , ak |ai i = ai , (i = 1, . . . , k), ai ai+1 = ai , (i = 1, . . . , k − 1)i,
1
it is clear that Gp(π) is a cyclic group of order pα
1 − 1. Our results on the
Sg(π) and M on(π) are the following:
Theorem A. For every integer n ≥ 4, the monoid M on(π) is of order n and
the semigroup Sg(π) is order n − 1. Moreover, M on(π) is non-commutative
and regular monoid but is not an inverse monoid, for every n.
Corollary B. For p1 = p2 = &middot; &middot; &middot; = pk = p, if m = α1 + α2 + &middot; &middot; &middot; + αk is a
partition of the integer m ≥ 2, M on(π) is a p-monoid (monoids of order pm ).
Moreover, all the different monoids are non-isomorphic for all partitions of
m.
2. The proofs
In this section, we prove Theorem A and Corollary B.
l
Proof of Theorem A. First we show that the relation ali am
j = ai holds for
every positive integers l and m, where 1 ≤ i &lt; j ≤ k. Since ai ai+1 = ai then
31
Non-commutative finite monoids of a given order n ≥ 4
l
we get ali am
i+1 = ai . Therefore,
l
m
l
m
l
m
ali am
j = ai ai+1 aj = ai ai+1 ai+2 aj = &middot; &middot; &middot; = ai ai+1 ai+2 &middot; &middot; &middot; aj−2 aj−1 aj
= ali ai+1 ai+2 &middot; &middot; &middot; aj−2 aj−1
= ali ai+1 ai+2 &middot; &middot; &middot; aj−2
= &middot;&middot;&middot;
= ali ai+1 ai+2
= ali ai+1
= ali .
This implies that every elements of M on(π) may be uniquely presented in
tk−1
i
the form atkk ak−1
&middot; &middot; &middot; at11 , where 0 ≤ ti ≤ pα
i − 1. Thus M on(π) is of order
αk
α1 α2
p1 p2 &middot; &middot; &middot; pk = n.
tk−1
To prove the regularity of the monoid M on(π), let w = atkk ak−1
&middot; &middot; &middot; atii be
an arbitrary element of M on(π), where i is the least positive integer such that
ti 6= 0. Two cases occur:
α
pi i −ti −1
0
i
. So,
Case 1. If ti + 1 &lt; pα
i then we set w = ai
p
t
αi
k−1
ww0 w = (atkk ak−1
&middot; &middot; &middot; atii )ai i
t
p
αi
−1
t
p
αi
−1
t
p
αi
+t
k−1
= atkk ak−1
&middot; &middot; &middot; ai i
k−1
= atkk ak−1
&middot; &middot; &middot; ai i
k−1
= atkk ak−1
&middot; &middot; &middot; ai i
t
k−1
= atkk ak−1
&middot; &middot; &middot; atii
= w.
−ti −1
t
t
k−1
(atkk ak−1
&middot; &middot; &middot; atii )
k−1
atkk ak−1
&middot; &middot; &middot; atii
atii
i −1
Non-commutative finite monoids of a given order n ≥ 4
p
αi
−1
0
i
i
Case 2. If ti + 1 = pα
i we may set w = ai
α
pi i −1
t
k−1
ww0 w = (atkk ak−1
&middot; &middot; &middot; atii )ai
p
t
αi
+t
k−1
= atkk ak−1
&middot; &middot; &middot; ai i
t
i −1
32
. Then,
t
k−1
(atkk ak−1
&middot; &middot; &middot; atii )
t
k−1
atkk ak−1
&middot; &middot; &middot; atii
t
k−1
k−1
= atkk ak−1
&middot; &middot; &middot; atii atkk ak−1
&middot; &middot; &middot; atii
t
k−1
= atkk ak−1
&middot; &middot; &middot; atii atii
t
k−1
= atkk ak−1
&middot; &middot; &middot; atii +1 atii −1
p
t
αi
k−1
= atkk ak−1
&middot; &middot; &middot; ai i atii −1
t
k−1
= atkk ak−1
&middot; &middot; &middot; atii
= w.
Every inverse monoid is indeed a regular monoid however, the converse is
not the case in general. Here we show that the monoid M on(π) is not an
inverse monoid, to do this we take into consideration three cases:
i
Case 1. There exists i, (1 ≤ i ≤ k − 1), such that pα
i &gt; 2. Then the element
p
αi
−2
α
i+1
ai+1 ai has the inverses ati+1 ai i
, where 0 ≤ t ≤ pi+1
− 1.
αk
αi
Case 2. For every i, (1 ≤ i ≤ k − 1), pi = 2 and pk &gt; 2. Then the element
αi+1
ai+1 ai has the inverses ati+1 ai , where 0 ≤ t ≤ pi+1
− 1.
αi
Case 3. For every i, (1 ≤ i ≤ k), pi = 2. Then the element a1 has the
inverses a1 and a2 a1 . This shows that there are different inverses for some
elements of M on(π).
Proof of Corollary B. The first part is a straightforward result of the proof
of Theorem A. To prove the second part, let m = α1 + α2 + &middot; &middot; &middot; + αk and
m = β1 + β2 + &middot; &middot; &middot; + βl be different partitions of the integer m ≥ 2, where k ≥ 2
and l ≥ 2. Also, let
π1 = ha1 , a2 , . . . , ak |api
αi
= ai , (1 ≤ i ≤ k), ai ai+1 = ai , (1 ≤ i ≤ k − 1)i,
and
π2 = hb1 , b2 , . . . , bl |bpj
βj
= bj , (1 ≤ j ≤ l), bj bj+1 = bj , (1 ≤ j ≤ l − 1)i,
j=l
be the related presentations for the partitions (αi )i=k
i=1 and (βj )j=1 , respectively.
If k 6= l, then the number of generators in M on(π1 ) and M on(π2 ) is not equal
and hence the monoids are non-isomorphic. Now suppose k = l. Since these
partitions are different and have the same length there exists an integer r,
(1 ≤ r ≤ k), such that αr 6= βj , for every j, (1 ≤ j ≤ k). Now if f :
tk−1
M on(π1 ) −→ M on(π2 ) is a monoid isomorphism and f (ar ) = btkk bk−1
&middot; &middot; &middot; bt11 ,
33
Non-commutative finite monoids of a given order n ≥ 4
where 0 ≤ tj ≤ pβj − 1, then the period(ar ) is equal to the period(f (ar )). This
implies αr = βs , for an s, (1 ≤ s ≤ k), which is a contradiction.
3. Remarks
t
t
t
j−1
i+1 ti
ai be an arbitrary non-identity element
&middot; &middot; &middot; ai+1
Remark 1. Let w = ajj aj−1
of M on(π), where i is the least positive integer such that ti 6= 0 and j is the
greatest positive integer such that tj 6= 0, (1 ≤ i ≤ j ≤ k). Then,
t
t
t
t0
j−1
i+1
i
aii |1 ≤ t0i ≤ pα
CM on(π) (w) = {ajj aj−1
&middot; &middot; &middot; ai+1
i − 1} ∪ {1}.
t0
t0
t0
s−1
Proof. Let w0 = ass as−1
&middot; &middot; &middot; arr be a non-identity element of M on(π), where
r is the least positive integer such that t0r 6= 0 and s is the greatest positive
integer such that t0s 6= 0, (1 ≤ r ≤ s ≤ k). If ww0 = w0 w, then
t
t0
t
t0
t0
t0
t0
t0
t
t
j−1
j−1
s−1
s−1
(ajj aj−1
&middot; &middot; &middot; atii )(ass as−1
&middot; &middot; &middot; arr ) = (ass as−1
&middot; &middot; &middot; arr )(ajj aj−1
&middot; &middot; &middot; atii ).
Three cases occur:
Case 1. i &gt; s. Then,
t
t0
t
t0
t0
t0
t0
t0
j−1
s−1
s−1
(ajj aj−1
&middot; &middot; &middot; atii )(ass as−1
&middot; &middot; &middot; arr ) = ass as−1
&middot; &middot; &middot; arr .
t
k−1
Since every elements of M on(π) have unique presentation as atkk ak−1
&middot; &middot; &middot; at11 ,
it is necessary that tj = tj−1 = &middot; &middot; &middot; = ti = 0, which is a contradiction with
w 6= 1.
Case 2. There exists l, (r &lt; l ≤ s) such that i = l. Then,
t
0
t +t0i tl−1
al−1
t
j−1
ajj aj−1
&middot; &middot; &middot; aii
t0
t0
t0
0
t0 t
t0
s−1
l−1
&middot; &middot; &middot; arr = ass as−1
&middot; &middot; &middot; aii al−1
&middot; &middot; &middot; arr .
Therefore, ti = 0, which is a contradiction.
Case 3. i = r. Then,
t
t
t +t0i
j−1
ajj aj−1
&middot; &middot; &middot; aii
t0
t0
t0 +ti
s−1
= ass as−1
&middot; &middot; &middot; aii
.
Hence, j = s and tl = t0l , for every l, (j ≤ l ≤ i − 1). Consequently, w0 =
t
t
t
t0
j−1
i+1
ajj aj−1
&middot; &middot; &middot; ai+1
aii . This completes the proof.
As an important result of Remark 1 we get:
Remark 2. M on(π) is centerless. So, there exist finite p-monoids which have
the trivial center. (In spite of the fact that finite p-groups have non-trivial
center.)
Non-commutative finite monoids of a given order n ≥ 4
34
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Non-commutative finite monoids of a given order n ≥ 4
Department of Mathematics,
Lahijan Branch,
Email: [email protected], [email protected]
Colin M. CAMPBELL,
School of Mathematics and Statistics,
University of St. Andrews,
North Haugh, St. Andrews, Fife KY16 9SS, Scotland, UK.
Email: [email protected]
Hossein DOOSTIE,
Department of Mathematics,
Science and Research Branch,
Islamic Azad University, Tehran, P.O. Box 14515/1775, Iran.
Email: [email protected]
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Non-commutative finite monoids of a given order n ≥ 4
36
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