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# counting number of matrices in finite field

I am new to sage and I want to count the number of matrices per rank in a given finite field.

Example: M=matrix([[x1, x2, x3],[ x4,-x1, x5],[x6,x7,x8]]) in Z/3Z

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## 1 Answer

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First, you can define the finite with 3 elements as follows:

sage: F = GF(3)
sage: F
Finite Field of size 3


The very cool thing in Sage is that you can define parents (check the doc) and iterate over its elements. So, we can construct the matrix space of 3 by 3 matrices over the field F:

sage: M = MatrixSpace(F,3,3)
sage: M
Full MatrixSpace of 3 by 3 dense matrices over Finite Field of size 3


Then we can look at the list of possible matrices (warning, the following produces a large output):

sage: list(M)


To count the number of matrices per rank, you can use a counter and iterate over all matrices (by incrementing the entry of the counter that corresponds to the rank of the matrix):

sage: from collections import Counter
sage: c = Counter()
sage: for m in M:
....:     c[m.rank()] += 1


Then you can check that there are 11232 matrices of rank 3, and do on.

sage: c
Counter({3: 11232, 2: 8112, 1: 338, 0: 1})

sage: c
8112


All that said, note that there are formulas to provide such numbers without having to iterate over all matrices (since at least [Landsberg 1893]), but i guess this was not the question.

EDIT

If you want to iterate over some particular matrices like matrices of the form matrix([[x1, x2, x3],[ x4,-x1, x5],[x6,x7,x8]]), you can use the product F^8 of F by itself 8 times using itertools:

sage: from collections import Counter
sage: c = Counter()

sage: from itertools import product
sage: P = product(F,repeat=8)

sage: for x in P:
....:     c[matrix([[x, x, x],[x,-x, x],[x,x,x]]).rank()] += 1

sage: c
Counter({3: 3780, 2: 2658, 1: 122, 0: 1})


Note that in Pyhon (hence Sage) the indices start at 0, not 1.

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## Comments

Thank you for answering my question. I am trying to count them on modules like M=matrix([[x1,x2],[x3,-x1]]) or M=matrix([x1,x2,x3],[x4,x5,x6],[x7,x8,-x1-x5]]) Is there a way where I can put the variables and make sage run them through GF(p)

1

How about something like this: L = [matrix(2, 2, [x, y, z, -x]) for x in GF(3) for y in GF(3) for z in GF(3)]?

1

@john-palmieri storing all matrices in a list might cost some memory (unless you plan to reuse them), you should better iterate over them.

1

@smokzeta i edited my answer to answer your question, i did not see the -x1 and though you want to iterate over all matrices.

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Asked: 2020-12-02 16:21:39 +0200

Seen: 80 times

Last updated: Dec 03 '20