Difference between revisions of "Unitriangular matrix group:UT(3,p)"
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Revision as of 17:59, 27 March 2012
This article is about a family of groups with a parameter that is prime. For any fixed value of the prime, we get a particular group.
View other such primeparametrized groups
Contents
Definition
As a group of matrices
Given a prime , the group is defined as follows: it is the group of upper triangular matrices with 1s on the diagonal, and entries over (with the group operation being matrix multiplication).
Each such matrix can be described by the three entries . The matrix looks like:
The multiplication of matrices and gives the matrix where:
The analysis given below does not apply to the case . For , we get the dihedral group:D8, which is studied separately.
As a semidirect product
This group of order can also be described as a semidirect product of the elementary abelian group of order by the cyclic group of order , where the generator of the cyclic group of order acts via the automorphism:
In this case, for instance, we can take the subgroup with as the elementary abelian subgroup of order and the subgroup with as the cyclic subgroup of order .
Families
 These groups fall in the more general family of unipotent uppertriangular matrices, which is the Sylow subgroup of the general linear group . This further can be generalized to where is the power of a prime , which is the Sylow subgroup of .
 These groups also fall into the general family of extraspecial groups.
Elements
Upto conjugacy
Every element has order .
The conjugacy classes are as follows:
 The center has order precisely , so there are elements that form conjugacy classes of size 1. These are, specifically, the elements with , and they're thus parametrized by their entry.
 For every element outside the center, the centralizer of that element is the subgroup generated by that element and the center, and is hence of order . Thus, the conjugacy class of the element is of size . Thus, there is a total of conjugacy classes of size .
Upto automorphism
There are only three classes of elements upto automorphism:
 The identity element, which forms a class of size 1
 The nonidentity elements in the center, which form a class of size
 The noncentral elements, which form a class of size
Arithmetic functions
Compare and contrast arithmetic function values with other groups of primecube order at Groups of primecube order#Arithmetic functions
For some of these, the function values are different when and/or when . These are clearly indicated below.
Arithmetic functions taking values between 0 and 3
Function  Value  Explanation 

primebase logarithm of order  3  the order is 
primebase logarithm of exponent  1  the exponent is . Exception when , where the exponent is . 
nilpotency class  2  
derived length  2  
Frattini length  2  
minimum size of generating set  2  
subgroup rank  2  
rank as pgroup  2  
normal rank as pgroup  2  
characteristic rank as pgroup  1 
Arithmetic functions of a counting nature
Function  Value  Explanation 

number of conjugacy classes  elements in the center, and each other conjugacy class has size  
number of subgroups  when , when  
number of normal subgroups  
number of conjugacy classes of subgroups  for , for 
Subgroups
Further information: Subgroup structure of primecube order group:U(3,p) Here is the complete list of subgroups:
 The trivial subgroup (1)
 The center, which is a group of order . In matrix terms, this is the subgroup comprising matrices with . (1)
 Subgroups of order generated by noncentral elements. These are not normal, and occur in conjugacy classes of size . ()
 Subgroups of order containing the center. These are the inverse images via the quotient map by the center, of subgroups of order in the inner automorphism group. ()
 The whole group. (1)
Normal subgroups
The subgroups in (1), (2), (4) and (5) above are normal.
Characteristic subgroups
The subgroups in (1), (2) and (5) above are normal. In other words, there are only three characteristic subgroups. Some notable facts:
 The group is characteristiccomparable: any two characteristic subgroups can be compared
 More generally, any characteristic subgroup and any normal subgroup can be compared.
 The characteristic subgroups are precisely the subgroups that occur in the derived series, upper central series and lower central series.
Subgroupdefining functions
Subgroupdefining function  Subgroup type in list  Isomorphism class  Comment 

Center  (2)  Group of prime order  
Commutator subgroup  (2)  Group of prime order  
Frattini subgroup  (2)  Group of prime order  The maximal subgroups of order intersect here. 
Socle  (2)  Group of prime order  This subgroup is the unique minimal normal subgroup, i.e.,the monolith, and the group is monolithic. Also, minimal normal implies central in nilpotent. 
Quotientdefining function
Quotientdefining function  Isomorphism class  Comment 

Inner automorphism group  Elementary abelian group of primesquare order  It is the quotient by the center, which is of prime order. 
Abelianization  Elementary abelian group of primesquare order  It is the quotient by the commutator subgroup, which is of prime order. 
Frattini quotient  Elementary abelian group of primesquare order  It is the quotient by the Frattini subgroup, which is of prime order. 
GAP implementation
GAP ID
For any prime , this group is the third group among the groups of order . Thus, for instance, if , the group is described using GAP's SmallGroup function as:
SmallGroup(343,3)
Note that we don't need to compute ; we can also write this as:
SmallGroup(7^3,3)
As an extraspecial group
For any prime , we can define this group using GAP's ExtraspecialGroup function as:
ExtraspecialGroup(p^3,'+')
For , it can also be constructed as:
ExtraspecialGroup(p^3,p)
where the argument indicates that it is the extraspecial group of exponent . For instance, for :
ExtraspecialGroup(5^3,5)
Endomorphisms
Automorphisms
The automorphisms essentially permute the subgroups of order containing the center, while leaving the center itself unmoved.
Related groups
For any prime , there are (up to isomorphism) two nonabelian groups of order . One of them is this, and the other is the semidirect product of the cyclic group of order by a group of order acting by power maps (with the generator corresponding to multiplication by ).