Element structure of unitriangular matrix group:UT(3,p)

This article describes in detail the element structure of the unitriangular matrix group:UT(3,p) for a prime number $$p$$. This is a special case of unitriangular matrix group of degree three over a finite field. For the element structure of this larger family, see element structure of unitriangular matrix group of degree three over a finite field.

The case $$p = 2$$, which gives dihedral group:D8, behaves somewhat differently from the case $$p$$ odd.

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 the field size $$q$$ (which here equals the prime $$p$$) of degree 3 - 1 = 2. Indeed, this is the case, and the explicit polynomial is $$p^2 + p - 1$$.

Conjugacy class structure in the unitriangular matrix group
 Note that the characteristic polynomial of all elements in this group is $$(t - 1)^3$$, hence we do not devote a column to the characteristic polynomial.

For reference, we consider matrices of the form:

$$\begin{pmatrix} 1 & a_{12} & a_{13} \\ 0 & 1 & a_{23} \\ 0 & 0 & 1 \\\end{pmatrix}$$



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: