# Similarity class types of matrices with entries in a finite field¶

The notion of a matrix conjugacy class type was introduced by J. A. Green in [Green55], in the context of computing the irreducible characters of finite general linear groups. The class types are equivalence classes of similarity classes of square matrices with entries in a finite field which, roughly speaking, have the same qualitative properties.

For example, all similarity classes of the same class type have centralizers of the same cardinality and the same degrees of elementary divisors. Qualitative properties of similarity classes such as semisimplicity and regularity descend to class types.

The most important feature of similarity class types is that, for any $$n$$, the number of similarity class types of $$n\times n$$ matrices is independent of $$q$$. This makes it possible to perform many combinatorial calculations treating $$q$$ as a formal variable.

In order to define similarity class types, recall that similarity classes of $$n\times n$$ matrices with entries in $$\GF{q}$$ correspond to functions

$c: \mathrm{Irr}\GF{q[t]} \to \Lambda$

such that

$\sum_{f\in \mathrm{Irr}\GF{q[t]}} |c(f)|\deg f = n,$

where we denote the set of irreducible monic polynomials in $$\GF{q[t]}$$ by $$\mathrm{Irr}\GF{q[t]}$$, the set of all partitions by $$\Lambda$$, and the size of $$\lambda \in \Lambda$$ by $$|\lambda|$$.

Similarity classes indexed by functions $$c_1$$ and $$c_2$$ as above are said to be of the same type if there exists a degree-preserving self-bijection $$\sigma$$ of $$\mathrm{Irr}\GF{q[t]}$$ such that $$c_2 = c_1\circ \sigma$$. Thus, the type of $$c$$ remembers only the degrees of the polynomials (and not the polynomials themselves) for which $$c$$ takes a certain value $$\lambda$$. Replacing each irreducible polynomial of degree $$d$$ for which $$c$$ takes a non-trivial value $$\lambda$$ by the pair $$(d, \lambda)$$, we obtain a multiset of such pairs. Clearly, $$c_1$$ and $$c_2$$ have the same type if and only if these multisets are equal. Thus a similarity class type may be viewed as a multiset of pairs of the form $$(d, \lambda)$$.

For $$2 \times 2$$ matrices there are four types:

sage: for tau in SimilarityClassTypes(2):
....:    print(tau)
[[1, ], [1, ]]
[[1, ]]
[[1, [1, 1]]]
[[2, ]]


These four types correspond to the regular split semisimple matrices, the non-semisimple matrices, the central matrices and the irreducble matrices respectively.

For any matrix $$A$$ in a given similarity class type, it is possible to calculate the number elements in the similarity class of $$A$$, the dimension of the algebra of matrices in $$M_n(A)$$ that commute with $$A$$, and the cardinality of the subgroup of $$GL_n(\GF{q})$$ that commute with $$A$$. For each similarity class type, it is also possible to compute the number of classes of that type (and hence, the total number of matrices of that type). All these calculations treat the cardinality $$q$$ of the finite field as a formal variable:

sage: M = SimilarityClassType([[1, ], [1, ]])
sage: M.class_card()
q^2 + q
sage: M.centralizer_algebra_dim()
2
sage: M.centralizer_group_card()
q^2 - 2*q + 1
sage: M.number_of_classes()
1/2*q^2 - 1/2*q
sage: M.number_of_matrices()
1/2*q^4 - 1/2*q^2


We now describe two applications of similarity class types.

We say that an $$n \times n$$ matrix has rational canonical form type $$\lambda$$ for some partition $$\lambda$$ of $$n$$ if the diagonal blocks in the rational canonical form have sizes given by the parts of $$\lambda$$. Thus the matrices with rational canonical type $$(n)$$ are the regular ones, while the matrices with rational canonical type $$(1^n)$$ are the central ones.

Using similarity class types, it becomes easy to get a formula for the number of matrices with a given rational canonical type:

sage: def matrices_with_rcf(la):
....:    return sum([tau.number_of_matrices() for tau in filter(lambda tau:tau.rcf()==la, SimilarityClassTypes(la.size()))])
sage: matrices_with_rcf(Partition([2,1]))
q^6 + q^5 + q^4 - q^3 - q^2 - q


Similarity class types can also be used to calculate the number of simultaneous similarity classes of $$k$$-tuples of $$n\times n$$ matrices with entries in $$\GF{q}$$ by using Burnside’s lemma:

sage: from sage.combinat.similarity_class_type import order_of_general_linear_group, centralizer_algebra_dim
sage: q = ZZ['q'].gen()
sage: def simultaneous_similarity_classes(n,k):
....:     return SimilarityClassTypes(n).sum(lambda la: q**(k*centralizer_algebra_dim(la)), invertible = True)/order_of_general_linear_group(n)
sage: simultaneous_similarity_classes(3, 2)
q^10 + q^8 + 2*q^7 + 2*q^6 + 2*q^5 + q^4


Similarity class types can be used to calculate the coefficients of generating functions coming from the cycle index type techniques of Kung and Stong (see Morrison [Morrison06]).

Along with the results of [PSS13], similarity class types can be used to calculate the number of similarity classes of matrices of order $$n$$ with entries in a principal ideal local ring of length two with residue field of cardinality $$q$$ with centralizer of any given cardinality up to $$n = 4$$. Among these, the classes which are selftranspose can also be counted:

sage: from sage.combinat.similarity_class_type import matrix_centralizer_cardinalities_length_two
sage: list(matrix_centralizer_cardinalities_length_two(3))
[(q^6 - 3*q^5 + 3*q^4 - q^3, 1/6*q^6 - 1/2*q^5 + 1/3*q^4),
(q^6 - 2*q^5 + q^4, q^5 - q^4),
(q^8 - 3*q^7 + 3*q^6 - q^5, 1/2*q^5 - q^4 + 1/2*q^3),
(q^8 - 2*q^7 + q^6, q^4 - q^3),
(q^10 - 2*q^9 + 2*q^7 - q^6, q^4 - q^3),
(q^8 - q^7 - q^6 + q^5, 1/2*q^5 - q^4 + 1/2*q^3),
(q^6 - q^5 - q^4 + q^3, 1/2*q^6 - 1/2*q^5),
(q^6 - q^5, q^4),
(q^10 - 2*q^9 + q^8, q^3),
(q^8 - 2*q^7 + q^6, q^4 - q^3),
(q^8 - q^7, q^3 + q^2),
(q^12 - 3*q^11 + 3*q^10 - q^9, 1/6*q^4 - 1/2*q^3 + 1/3*q^2),
(q^12 - 2*q^11 + q^10, q^3 - q^2),
(q^14 - 2*q^13 + 2*q^11 - q^10, q^3 - q^2),
(q^12 - q^11 - q^10 + q^9, 1/2*q^4 - 1/2*q^3),
(q^12 - q^11, q^2),
(q^14 - 2*q^13 + q^12, q^2),
(q^18 - q^17 - q^16 + q^14 + q^13 - q^12, q^2),
(q^12 - q^9, 1/3*q^4 - 1/3*q^2),
(q^6 - q^3, 1/3*q^6 - 1/3*q^4)]


REFERENCES:

 [Green55] Green, J. A. The characters of the finite general linear groups. Trans. Amer. Math. Soc. 80 (1955), 402–447. doi:10.1090/S0002-9947-1955-0072878-2
 [Morrison06] Morrison, Kent E. Integer sequences and matrices over finite fields. J. Integer Seq. 9 (2006), no. 2, Article 06.2.1, 28 pp. https://cs.uwaterloo.ca/journals/JIS/VOL9/Morrison/morrison37.html
 [PSS13] (1, 2) Prasad, A., Singla, P., and Spallone, S., Similarity of matrices over local rings of length two. arXiv 1212.6157

AUTHOR:

• Amritanshu Prasad (2013-07-18): initial implementation
• Amritanshu Prasad (2013-09-09): added functions for similarity classes over rings of length two
class sage.combinat.similarity_class_type.PrimarySimilarityClassType(parent, deg, par)

A primary similarity class type is a pair consisting of a partition and a positive integer.

For a partition $$\lambda$$ and a positive integer $$d$$, the primary similarity class type $$(d, \lambda)$$ represents similarity classes of square matrices of order $$|\lambda| \cdot d$$ with entries in a finite field of order $$q$$ which correspond to the $$\GF{q[t]}$$-module

$\frac{\GF{q[t]}}{p(t)^{\lambda_1} } \oplus \frac{\GF{q[t]}}{p(t)^{\lambda_2}} \oplus \dotsb$

for some irreducible polynomial $$p(t)$$ of degree $$d$$.

centralizer_algebra_dim()

Return the dimension of the algebra of matrices which commute with a matrix of type self.

For a partition $$(d, \lambda)$$ this dimension is given by $$d(\lambda_1 + 3\lambda_2 + 5\lambda_3 + \cdots)$$.

EXAMPLES:

sage: PT = PrimarySimilarityClassType(2, [3, 2, 1])
sage: PT.centralizer_algebra_dim()
28

centralizer_group_card(q=None)

Return the cardinality of the centralizer group of a matrix of type self in a field of order q.

INPUT:

• q – an integer or an indeterminate

EXAMPLES:

sage: PT = PrimarySimilarityClassType(1, [])
sage: PT.centralizer_group_card()
1
sage: PT = PrimarySimilarityClassType(2, [1, 1])
sage: PT.centralizer_group_card()
q^8 - q^6 - q^4 + q^2

degree()

Return degree of self.

EXAMPLES:

sage: PT = PrimarySimilarityClassType(2, [3, 2, 1])
sage: PT.degree()
2

partition()

Return partition corresponding to self.

EXAMPLES:

sage: PT = PrimarySimilarityClassType(2, [3, 2, 1])
sage: PT.partition()
[3, 2, 1]

size()

Return the size of self.

EXAMPLES:

sage: PT = PrimarySimilarityClassType(2, [3, 2, 1])
sage: PT.size()
12

statistic(func, q=None)

Return $$n_{\lambda}(q^d)$$ where $$n_{\lambda}$$ is the value returned by func upon input $$\lambda$$, if self is $$(d, \lambda)$$.

EXAMPLES:

sage: PT = PrimarySimilarityClassType(2, [3, 1])
sage: q = ZZ['q'].gen()
sage: PT.statistic(lambda la:q**la.size(), q = q)
q^8

class sage.combinat.similarity_class_type.PrimarySimilarityClassTypes(n, min)

All primary similarity class types of size n whose degree is greater than that of min or whose degree is that of min and whose partition is less than of min in lexicographic order.

A primary similarity class type of size $$n$$ is a pair $$(\lambda, d)$$ consisting of a partition $$\lambda$$ and a positive integer $$d$$ such that $$|\lambda| d = n$$.

INPUT:

• n – a positive integer
• min – a primary matrix type of size n

EXAMPLES:

If min is not specified, then the class of all primary similarity class types of size n is created:

sage: PTC = PrimarySimilarityClassTypes(2)
sage: for PT in PTC:
....:     print(PT)
[1, ]
[1, [1, 1]]
[2, ]


If min is specified, then the class consists of only those primary similarity class types whose degree is greater than that of min or whose degree is that of min and whose partition is less than of min in lexicographic order:

sage: PTC = PrimarySimilarityClassTypes(2, min = PrimarySimilarityClassType(1, [1, 1]))
sage: for PT in PTC:
....:     print(PT)
[1, [1, 1]]
[2, ]

Element
size()

Return size of elements of self.

The size of a primary similarity class type $$(d, \lambda)$$ is $$d |\lambda|$$.

EXAMPLES:

sage: PTC = PrimarySimilarityClassTypes(2)
sage: PTC.size()
2

class sage.combinat.similarity_class_type.SimilarityClassType(parent, tau)

A similarity class type.

A matrix type is a multiset of primary similarity class types.

INPUT:

• tau – A list of primary similarity class types

EXAMPLES:

sage: tau1 = SimilarityClassType([[3, [3, 2, 1]], [2, [2, 1]]]); tau1
[[2, [2, 1]], [3, [3, 2, 1]]]

as_partition_dictionary()

Return a dictionary whose keys are the partitions of types occuring in self and the value at the key $$\lambda$$ is the partition formed by sorting the degrees of primary types with partition $$\lambda$$.

EXAMPLES:

sage: tau = SimilarityClassType([[1, ], [1, ]])
sage: tau.as_partition_dictionary()
{: [1, 1]}

centralizer_algebra_dim()

Return the dimension of the algebra of matrices which commute with a matrix of type self.

EXAMPLES:

sage: tau = SimilarityClassType([[1, ], [1, ]])
sage: tau.centralizer_algebra_dim()
2

centralizer_group_card(q=None)

Return the cardinality of the group of matrices in $$GL_n(\GF{q})$$ which commute with a matrix of type self.

INPUT:

• q – an integer or an indeterminate

EXAMPLES:

sage: tau = SimilarityClassType([[1, ], [1, ]])
sage: tau.centralizer_group_card()
q^2 - 2*q + 1

class_card(q=None)

Return the number of matrices in each similarity class of type self.

INPUT:

• q – an integer or an indeterminate

EXAMPLES:

sage: tau = SimilarityClassType([[1, [1, 1, 1, 1]]])
sage: tau.class_card()
1
sage: tau = SimilarityClassType([[1, ], [1, ]])
sage: tau.class_card()
q^2 + q

is_regular()

Return True if every primary type in self has partition with one part.

EXAMPLES:

sage: tau = SimilarityClassType([[2, ], [1, ]])
sage: tau.is_regular()
True
sage: tau = SimilarityClassType([[2, [1, 1]], [1, ]])
sage: tau.is_regular()
False

is_semisimple()

Return True if every primary similarity class type in self has all parts equal to 1.

EXAMPLES:

sage: tau = SimilarityClassType([[2, [1, 1]], [1, ]])
sage: tau.is_semisimple()
True
sage: tau = SimilarityClassType([[2, [1, 1]], [1, ]])
sage: tau.is_semisimple()
False

number_of_classes(invertible=False, q=None)

Return the number of similarity classes of matrices of type self.

INPUT:

• invertible – Boolean; return number of invertible classes if set to True
• q – An integer or an indeterminate

EXAMPLES:

sage: tau = SimilarityClassType([[1, ], [1, ]])
sage: tau.number_of_classes()
1/2*q^2 - 1/2*q

number_of_matrices(invertible=False, q=None)

Return the number of matrices of type self.

INPUT:

• invertible – A boolean; return the number of invertible matrices if set

EXAMPLES:

sage: tau = SimilarityClassType([[1, ]])
sage: tau.number_of_matrices()
q
sage: tau.number_of_matrices(invertible = True)
q - 1
sage: tau = SimilarityClassType([[1, ], [1, ]])
sage: tau.number_of_matrices()
1/2*q^4 - 1/2*q^2

rcf()

Return the partition corresponding to the rational canonical form of a matrix of type self.

EXAMPLES:

sage: tau = SimilarityClassType([[2, [1, 1, 1]], [1, [3, 2]]])
sage: tau.rcf()
[5, 4, 2]

size()

Return the sum of the sizes of the primary parts of self.

EXAMPLES:

sage: tau = SimilarityClassType([[3, [3, 2, 1]], [2, [2, 1]]])
sage: tau.size()
24

statistic(func, q=None)

Return

$\prod_{(d, \lambda)\in \tau} n_{\lambda}(q^d)$

where $$n_{\lambda}(q)$$ is the value returned by func on the input $$\lambda$$.

INPUT:

• func – a function that takes a partition to a polynomial in q
• q – an integer or an indeterminate

EXAMPLES:

sage: tau = SimilarityClassType([[1, ], [1, [2, 1]], [2, [1, 1]]])
sage: from sage.combinat.similarity_class_type import fq
sage: tau.statistic(lambda la: prod([fq(m) for m in la.to_exp()]))
(q^9 - 3*q^8 + 2*q^7 + 2*q^6 - 4*q^5 + 4*q^4 - 2*q^3 - 2*q^2 + 3*q - 1)/q^9
sage: q = ZZ['q'].gen()
sage: tau.statistic(lambda la: q**la.size(), q = q)
q^8

class sage.combinat.similarity_class_type.SimilarityClassTypes(n, min)

Class of all similarity class types of size n with all primary matrix types greater than or equal to the primary matrix type min.

A similarity class type is a multiset of primary matrix types.

INPUT:

• n – a non-negative integer
• min – a primary similarity class type

EXAMPLES:

If min is not specified, then the class of all matrix types of size n is constructed:

sage: M = SimilarityClassTypes(2)
sage: for tau in M:
....:     print(tau)
[[1, ], [1, ]]
[[1, ]]
[[1, [1, 1]]]
[[2, ]]


If min is specified, then the class consists of only those similarity class types which are multisets of primary matrix types which either have size greater than that of min, or if they have size equal to that of min, then they occur after min in the iterator for PrimarySimilarityClassTypes(n), where n is the size of min:

sage: M = SimilarityClassTypes(2, min = [1, [1, 1]])
sage: for tau in M:
....:     print(tau)
[[1, [1, 1]]]
[[2, ]]

Element

alias of SimilarityClassType

size()

Return size of self.

EXAMPLES:

sage: tau = SimilarityClassType([[3, [3, 2, 1]], [2, [2, 1]]])
sage: tau.parent().size()
24

sum(stat, sumover='matrices', invertible=False, q=None)

Return the sum of a local statistic over all types.

Given a set of functions $$n_{\lambda}(q)$$ (these could be polynomials or rational functions in $$q$$, for each similarity class type $$\tau$$ define

$n_\tau(q) = \prod_{(d,\lambda)\in \tau} n_{\lambda}(q^d).$

This function returns

$\sum n_{\tau(g)}(q)$

where $$\tau(g)$$ denotes the type of a matrix $$g$$, and the sum is over all $$n \times n$$ matrices if sumover is set to "matrices", is over all $$n \times n$$ similarity classes if sumover is set to "classes", and over all $$n \times n$$ types if sumover is set to "types". If invertible is set to True, then the sum is only over invertible matrices or classes.

INPUT:

• stat – a function which takes partitions and returns a function of q
• sumover – can be one of the following:
• "matrices"
• "classes"
• "types"
• q – an integer or an indeterminate

OUTPUT:

A function of q.

EXAMPLES:

sage: M = SimilarityClassTypes(2)
sage: M.sum(lambda la:1)
q^4
sage: M.sum(lambda la:1, invertible = True)
q^4 - q^3 - q^2 + q
sage: M.sum(lambda la:1, sumover = "classes")
q^2 + q
sage: M.sum(lambda la:1, sumover = "classes", invertible = True)
q^2 - 1


Burside’s lemma can be used to calculate the number of similarity classes of matrices:

sage: from sage.combinat.similarity_class_type import centralizer_algebra_dim, order_of_general_linear_group
sage: q = ZZ['q'].gen()
sage: M.sum(lambda la:q**centralizer_algebra_dim(la), invertible = True)/order_of_general_linear_group(2)
q^2 + q

sage.combinat.similarity_class_type.centralizer_algebra_dim(la)

Return the dimension of the centralizer algebra in $$M_n(\GF{q})$$ of a nilpotent matrix whose Jordan blocks are given by la.

EXAMPLES:

sage: from sage.combinat.similarity_class_type import centralizer_algebra_dim
sage: centralizer_algebra_dim(Partition([2, 1]))
5


Note

If it is a list, la is expected to be sorted in decreasing order.

sage.combinat.similarity_class_type.centralizer_group_cardinality(la, q=None)

Return the cardinality of the centralizer group in $$GL_n(\GF{q})$$ of a nilpotent matrix whose Jordan blocks are given by la.

INPUT:

• lambda – a partition
• q – an integer or an indeterminate

OUTPUT:

A polynomial function of q.

EXAMPLES:

sage: from sage.combinat.similarity_class_type import centralizer_group_cardinality
sage: q = ZZ['q'].gen()
sage: centralizer_group_cardinality(Partition([2, 1]))
q^5 - 2*q^4 + q^3

sage.combinat.similarity_class_type.dictionary_from_generator(gen)

Given a generator for a list of pairs $$(c,f)$$, construct a dictionary whose keys are the distinct values for $$c$$ and whose value at $$c$$ is the sum of $$f$$ over all pairs of the form $$(c',f)$$ such that $$c=c'$$.

EXAMPLES:

sage: from sage.combinat.similarity_class_type import dictionary_from_generator
sage: dictionary_from_generator(((floor(x/2), x) for x in range(10)))
{0: 1, 1: 5, 2: 9, 3: 13, 4: 17}


It also works with lists:

sage: dictionary_from_generator([(floor(x/2),x) for x in range(10)])
{0: 1, 1: 5, 2: 9, 3: 13, 4: 17}


Note

Since the generator is first converted to a list, memory usage could be high.

sage.combinat.similarity_class_type.ext_orbit_centralizers(input_data, q=None, selftranspose=False)

Generate pairs consisting of centralizer cardinalities of orbits in $$\mathrm{Ext}^1(M, M)$$ for the action of $$\mathrm{Aut}(M, M)$$, where $$M$$ is the $$\GF{q[t]}$$-module constructed from input and their frequencies.

INPUT:

• input_data – input for input_parsing()
• q – (default: $$q$$) an integer or an indeterminate
• selftranspose – (default: False) boolean stating if we only want selftranspose type
sage.combinat.similarity_class_type.ext_orbits(input_data, q=None, selftranspose=False)

Return the number of orbits in $$\mathrm{Ext}^1(M, M)$$ for the action of $$\mathrm{Aut}(M, M)$$, where $$M$$ is the $$\GF{q[t]}$$-module constructed from input_data.

INPUT:

• input_data – input for input_parsing()
• q – (default: $$q$$) an integer or an indeterminate
• selftranspose – (default: False) boolean stating if we only want selftranspose type
sage.combinat.similarity_class_type.fq(n, q=None)

Return $$(1-q^{-1}) (1-q^{-2}) \cdots (1-q^{-n})$$.

INPUT:

• n – A non-negative integer
• q – an integer or an indeterminate

OUTPUT:

A rational function in q.

EXAMPLES:

sage: from sage.combinat.similarity_class_type import fq
sage: fq(0)
1
sage: fq(3)
(q^6 - q^5 - q^4 + q^2 + q - 1)/q^6

sage.combinat.similarity_class_type.input_parsing(data)

Recognize and return the intended type of input.

sage.combinat.similarity_class_type.matrix_centralizer_cardinalities(n, q=None, invertible=False)

Generate pairs consisting of centralizer cardinalities of matrices over a finite field and their frequencies.

sage.combinat.similarity_class_type.matrix_centralizer_cardinalities_length_two(n, q=None, selftranspose=False, invertible=False)

Generate pairs consisting of centralizer cardinalities of matrices over a principal ideal local ring of length two with residue field of order q and their frequencies.

INPUT:

• n – the order
• q – (default: $$q$$) an integer or an indeterminate
• selftranspose – (default: False) boolean stating if we only want selftranspose type
• invertible – (default: False) boolean stating if we only want invertible type
sage.combinat.similarity_class_type.matrix_similarity_classes(n, q=None, invertible=False)

Return the number of matrix similarity classes over a finite field of order q.

sage.combinat.similarity_class_type.matrix_similarity_classes_length_two(n, q=None, selftranspose=False, invertible=False)

Return the number of similarity classes of matrices of order n with entries in a principal ideal local ring of length two.

INPUT:

• n – the order
• q – (default: $$q$$) an integer or an indeterminate
• selftranspose – (default: False) boolean stating if we only want selftranspose type
• invertible – (default: False) boolean stating if we only want invertible type

EXAMPLES:

We can generate Table 6 of [PSS13]:

sage: from sage.combinat.similarity_class_type import matrix_similarity_classes_length_two
sage: matrix_similarity_classes_length_two(2)
q^4 + q^3 + q^2
sage: matrix_similarity_classes_length_two(2, invertible = True)
q^4 - q
sage: matrix_similarity_classes_length_two(3)
q^6 + q^5 + 2*q^4 + q^3 + 2*q^2
sage: matrix_similarity_classes_length_two(3, invertible = true)
q^6 - q^3 + 2*q^2 - 2*q
sage: matrix_similarity_classes_length_two(4)
q^8 + q^7 + 3*q^6 + 3*q^5 + 5*q^4 + 3*q^3 + 3*q^2
sage: matrix_similarity_classes_length_two(4, invertible = True)
q^8 + q^6 - q^5 + 2*q^4 - 2*q^3 + 2*q^2 - 3*q


And also Table 7:

sage: matrix_similarity_classes_length_two(2, selftranspose = True)
q^4 + q^3 + q^2
sage: matrix_similarity_classes_length_two(2, selftranspose = True, invertible = True)
q^4 - q
sage: matrix_similarity_classes_length_two(3, selftranspose = True)
q^6 + q^5 + 2*q^4 + q^3
sage: matrix_similarity_classes_length_two(3, selftranspose = True, invertible = True)
q^6 - q^3
sage: matrix_similarity_classes_length_two(4, selftranspose = True)
q^8 + q^7 + 3*q^6 + 3*q^5 + 3*q^4 + q^3 + q^2
sage: matrix_similarity_classes_length_two(4, selftranspose = True, invertible = True)
q^8 + q^6 - q^5 - q

sage.combinat.similarity_class_type.order_of_general_linear_group(n, q=None)

Return the cardinality of the group of $$n \times n$$ invertible matrices with entries in a field of order q.

INPUT:

• n – a non-negative integer
• q – an integer or an indeterminate

EXAMPLES:

sage: from sage.combinat.similarity_class_type import order_of_general_linear_group
sage: order_of_general_linear_group(0)
1
sage: order_of_general_linear_group(2)
q^4 - q^3 - q^2 + q

sage.combinat.similarity_class_type.primitives(n, invertible=False, q=None)

Return the number of similarity classes of simple matrices of order n with entries in a finite field of order q. This is the same as the number of irreducible polynomials of degree $$d$$.

If invertible is True, then only the number of similarity classes of invertible matrices is returned.

Note

All primitive classes are invertible unless n is $$1$$.

INPUT:

• n – a positive integer
• invertible – boolean; if set, only number of non-zero classes is returned
• q – an integer or an indeterminate

OUTPUT:

• a rational function of the variable q

EXAMPLES:

sage: from sage.combinat.similarity_class_type import primitives
sage: primitives(1)
q
sage: primitives(1, invertible = True)
q - 1
sage: primitives(4)
1/4*q^4 - 1/4*q^2
sage: primitives(4, invertible = True)
1/4*q^4 - 1/4*q^2