Element structure of unitriangular matrix group of degree three over a finite field
This article gives specific information, namely, element structure, about a family of groups, namely: unitriangular matrix group of degree three.
View element structure of group families  View other specific information about unitriangular matrix group of degree three
This article describes in detail the element structure of the unitriangular matrix group of degree three over a finite field. We denote the field size by , the field characteristic by , and the value by . Further, we denote the group as .
Summary
Item  Value 

number of conjugacy classes  
order  Agrees with general order formula for : 
conjugacy class size statistics  size 1 ( times), size ( times) 
order statistics  Case : order 1 (1 element), order 2 ( elements), order 4 ( elements) Case odd: order 1 (1 element), order ( elements) 
exponent  4 if if odd The exponent depends only on , not on . 
Conjugacy class structure
Number of conjugacy classes
The general theory says that number of conjugacy classes in unitriangular matrix group of fixed degree over a finite field is polynomial function of field size, where the degree of the polynomial is one less than the degree of matrices. Thus, we expect that the number of conjugacy classes is a polynomial function of of degree 3  1 = 2. Indeed, this is the case, and the explicit polynomial is .
Conjugacy class structure in the unitriangular matrix group
Note that the characteristic polynomial of all elements in this group is , hence we do not devote a column to the characteristic polynomial.
For reference, we consider matrices of the form:
Nature of conjugacy class  Jordan block size decomposition  Minimal polynomial  Size of conjugacy class  Number of such conjugacy classes  Total number of elements  Order of elements in each such conjugacy class  Type of matrix 

identity element  1 + 1 + 1 + 1  1  1  1  1  
nonidentity element, but central (has Jordan blocks of size one and two respectively)  2 + 1  1  ,  
noncentral, has Jordan blocks of size one and two respectively  2 + 1  , but not both and are zero  
noncentral, has Jordan block of size three  3  if odd 4 if 
both and are nonzero  
Total ()           
Grouping by conjugacy class sizes
Conjugacy class size  Total number of conjugacy classes of this size  Total number of elements  Cumulative number of conjugacy classes  Cumulative number of elements 

1  
(total)  (total) 
Conjugacy classes with respect to the general linear group
If we consider the action of the general linear group by conjugation, then there is considerable fusion of conjugacy classes. Specifically, there are only three equivalence classes, corresponding to the set of unordered integer partitions of 3 describing the possible Jordan block decompositions.
Below is a summary of the information:
Jordan block size decomposition (partition of 3)  Number of conjugacy classes of size 1 Number of elements in these 
Number of conjugacy classes of size Number of elements in these 
Total (number of conjugacy classes, elements) 

1 + 1 + 1  1 1 
0 0 
1 1 
2 + 1  


3  0 0 


Total ()  

