Steve Stavrou

## 60 Reputation

13 years, 230 days

## Generate invertible 4x4 matrices over F_...

Maple

Hi, here is the code I used to try to generate all invertible 4x4 matrices over the finite field F_2 = {0.1}. However, when I look at the elements of GROUP (see below) all the elements are 4x4 matrices with a 2 in each entry. I don't know why this is?

Also, I need help converting all of the invertible 4x4 matrices in the following way: I want the 4x4 matrices to each be written as a string of length 16 with no spaces, commas or brackets. So for example the matrix

a b c d

e f g h

i j k l

m n o p

becomes abcdefghijklmnop

restart:
with(LinearAlgebra):
PRIME := 2;
2

# the group of invertible 4 x 4 matrices over the field F_p

GROUP := []:
M := Matrix([[a,b,c,d],[x,f,g,h],[y,j,k,l],[m,n,o,p]]):
for a from 0 to PRIME-1 do
for b from 0 to PRIME-1 do
for c from 0 to PRIME-1 do
for d from 0 to PRIME-1 do
for x from 0 to PRIME-1 do
for f from 0 to PRIME-1 do
for g from 0 to PRIME-1 do
for h from 0 to PRIME-1 do
for y from 0 to PRIME-1 do
for j from 0 to PRIME-1 do
for k from 0 to PRIME-1 do
for l from 0 to PRIME-1 do
for m from 0 to PRIME-1 do
for n from 0 to PRIME-1 do
for o from 0 to PRIME-1 do
for p from 0 to PRIME-1 do
if Determinant(M) mod PRIME <> 0 then
GROUP := [ op(GROUP), M ]
fi
od od od od od od od od od od od od od od od od:

nops(GROUP);
20160

GROUP;

## How do I generate a list of all possible...

Maple

Let x and y be 4-digit integers such that the last digit of x is 7 and the last digit of y is 1. That is, x = abc7 and y = rst1, where a,b,c,r,s,t all run from 0 to 9. There are 1000 possibilities for x and  1000 for y. What are all possible products x*y? I would like all possible products listed in increasing order. The first element of the list should be 7*1 = 7 (since 0007*0001 = 7). The last should be 9997*9991 = 99880027. Thank you!

## How do I generate all 4x4 rank-1 matrice...

Maple

I want to generate all 4x4 matrices of rank-1 over the field with two elements, F_2. I don't want to use actual arrays though, I just want to use symbols representing the unit matrices. So for example Eij represents the 4x4 matrix with a 1 in row i & column j, and 0's elsewhere.

We have 16 unit matrices: E11, E12, ..., E14, ... E41, E42, E43, E44. Then we have to look at all possible linear combinations that result in a rank-1 matrix. So for example: E11+E12,  E11+E21,  E11+E12+E13,  E33+E43, etc. The total number of rank-1 4x4 matrices over F_2 is 225. How can I find all of these quickly?

## How do I classify elements into sets in ...

Maple 18

Hello!

I would like to start with the following set of 9 elements,
A = { E11, E12, E21, E22, E11+E12, E11+E21, E12+E22, E21+E22, E11+E12+E21+E22 }.

I need a procedure that takes each of those elements and creates 3 new ones in the following way: Eij becomes Eij1, Eij2, Eij1+Eij2. So for example, E11 will become: E111, E112, and E111+E112. And for example the fifth element in A (i.e. E11+E12) will become the 3 new elements: E111+E121, E112+E122, and E111+E121 + E112+E122.

Since each of the 9 elements gets triplicated, there will be a new set, call it B, with 27 elements.

B = {E111, E112, E111+E112, E121, E122, E121+E122, ... }

Now I want to repeat this process of triplicating again so that, for example, E111 becomes: E1111, E1112, and E1111+E1112. And so on. This new set C will have 81 elements. Now I want to repeat this one last time. The final set, D, will have 243 (3^5) elements.

Step 2:

For every pair of elements x and y in D, I want to compute z:=(x+y)mod2. If z already belongs to D, discard it, otherwise, place z in the set D2. Do this until there are no more elements to add together (note that if x+y is computed then I don't want y+x to be computed also--that's inefficient). Maybe the most efficient way is to perform all possibly combinations of x+y mod 2 to create the set D2 and then just go: D2 setminus D.

Step 3: For x in D and y in D2 perform all possible combinations of z:=(x+y)mod2 and place these in D3 then perform set subtraction again: D3 minus D2 minus D.

Repeat this process again: x in D and y in D3 to create new elements in D4. Repeat again until Dm is empty (that is, D(m-1) will be the last set that contains new elements). I'm expecting around 12 sets...

The issue with this whole algorithm is that I often run out of memory so I need a clever way to do this, since this algorithm is essentially classifying 2^32 elements into disjoint sets. Thank you!

## How do I get the elements to add togethe...

Maple

I have a set, R consisting of 9 elements:

R = {A_1, A_2, A_3, A_4, B_1, B_2, B_3, B_4, C_1}

where
B_1 = A_1 + A_2,
B_2 = A_1 + A_3,
B_3 = A_3 + A_4,
B_4 = A_2 + A_4,
C_1 = A_1 + A_2 + A_3 + A_4.

I want to know how to do 2 things:
1) For each element x in R I want x to become x_1, x_2, and x_3 (where x_3 = x_2 + x_1). So for example:
A_1 will become A_11, A_12, A_13 (where A_13 = A_11 + A_12). This should create a total of 27 elements (since each of the 9 elements splits into 3). I want all 27 elements to go into a new set called S_1.
2) The second thing I want is a procedure that takes x in S_1 and y in S_1 and computes z=(x+y)mod2. If z is not already an element of S_1 then put it into a new set S_2. This procedure should terminate when all the possible additions of x and y have been computed. I know that S_2 will contain 162 elements (all of which are different from the 27 elements in S_1). Then I want the procedure to take x in S_1 and y in S_2 and do the same thing: z=(x+y)mod2, where new elements not in S_1 or S_2 will be placed in S_3. Then again repeat this: x in S_1 and y in S_r until no new sets S_{r+1} can be created (i.e. S_{r+1} is empty because no new possible elements exist).

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