Sage is not returning all solutions to equations modulo n

asked 2015-06-24 18:05:16 +0100

Acanti
25 ●4

updated 2015-06-24 18:25:08 +0100

calc314
4191 ●22 ●48 ●113

I am trying to find all 2x2 matrices $S$ over $Z/9Z$ such that $S^3=I$, where I is the identity matrix. I am currently using the following procedure:

S = matrix(SR, 2, [[a,b],[c,d]]);
S3=S^3
l=solve_mod([S3[0,0]==1,S3[0,1]==0,S3[1,0]==0,S3[1,1]==1], 9);
l

The list of solutions (there are 207) I receive does not include S=[[1,3],[3,1]], for example, which does in fact satisfy $S^3=I$. I am new to Sage, is there something I am missing? How can I get a complete list of solutions?

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answered 2015-06-24 18:35:32 +0100

fidbc

2423 ●4 ●26 ●61

Trying to find them using brute force isn't that bad. The following worked for me

import itertools
Z9=Zmod(9)
I=Matrix(Z9,[[1,0],[0,1]])
L=[]
for a,b,c,d in itertools.product(Z9,repeat=4):
    M=Matrix(Z9,[[a,b],[c,d]])
    if I==M^3:
        L.append(M)

The length of the list seems to be 297 though, but the 39th element seems to be the one that was missing from your list.

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Thanks! This will fix my current problem, but I am still concerned for when I use larger matrices, and/or a different modulus. If anyone has ideas on a really efficient way to do this in general, I would love to hear them.

Acanti ( 2015-06-24 18:44:09 +0100 )edit

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Asked: 2015-06-24 18:05:16 +0100

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Last updated: Jun 24 '15

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