Ask Your Question
0

"how to test if two matrices are similar by a signed permutation matrix" code sage

asked 2020-02-22 20:44:59 +0100

imy gravatar image

"how to test if two matrices are similar by a signed permutation matrix" code sage

edit retag flag offensive close merge delete

Comments

What are the entries of the matrices? Real numbers or integers or?

rburing gravatar imagerburing ( 2020-02-23 00:10:38 +0100 )edit

The nature of the entries should not matter. For the problem to make sense you only need to be able to test for equality of entries and negates an entry.

vdelecroix gravatar imagevdelecroix ( 2020-02-23 10:27:57 +0100 )edit

the matrices are symmetric seidl matrices of simple graphs. zero on the diagonal , 1 and -1 off the diagonal

imy gravatar imageimy ( 2020-10-08 00:53:27 +0100 )edit

let A and B the seidel adjacency matrices of two simple graphs. How to write a code in sageMath for find a signed permutation matrix which gives A and B are similar

imy gravatar imageimy ( 2020-10-08 01:01:00 +0100 )edit

2 Answers

Sort by » oldest newest most voted
0

answered 2020-10-12 17:07:44 +0100

dan_fulea gravatar image

updated 2020-10-12 17:08:27 +0100

I gave a detailed answer to a fuzzy question by Anonymous in the link below:

https://ask.sagemath.org/question/39515/how-to-find-the-permutation-matrix-associated-with-the-similarity-transformation-in-the-following-code/

(But compared to this question, the 39515 question qualifies as a detailed exposition.)

The steps would be:

  • check the characteristic polynoials, if there is no match, we can stop here.
  • build the eigenvalues, sort them somehow, if there is no match, stop here.
  • build the jordan decomposition and match the eigenvectors, allowing rescaling on the one side, and permutations of rows on the other side. A rough match can be done by first rescaling in the two Jordan base change matrices, so that the maximal absolute value occurs for the entry $1$ in each column.
edit flag offensive delete link more
0

answered 2020-02-23 10:48:10 +0100

vdelecroix gravatar image

updated 2020-02-23 10:58:14 +0100

If you remove "signed" from your question this is equivalent to the isomorphism problem of vertex and edge labeled directed graphs. To do that in Sage let us define the following auxilliary function that turns a matrix into a (edge labeled) directed graph

def mat_to_dig(A):
    n = A.nrows()
    G = DiGraph(loops=False, multiedges=False)
    for i in range(n):
        for j in range(n):
            if A[i,j]:
                G.add_edge(i, j, A[i,j])
    return G

Now, (assuming that I care about matrix with 0 on the diagonal)

sage: A = matrix(4, [0,1,2,3,4,0,5,6,7,8,0,9,10,11,12,0])
sage: A
[ 0   1   2  3]
[ 4   0   5  6]
[ 7   8   0  9]
[10 11 12  0]
sage: p = Permutation('(1,3)(2,4)')
sage: B = copy(A)
sage: B.permute_rows_and_columns(p,p)
sage: B
[ 0   9   7  8]
[12  0 10 11]
[ 2   3   0  1]
[ 5   6   4  0]
sage: G = mat_to_dig(A)
sage: H = mat_to_dig(B)
sage: G.is_isomorphic(H, edge_labels=True, certificate=True)
(True, {0: 2, 1: 3, 2: 0, 3: 1})

The dictionary {0: 2, 1: 3, 2: 0, 3: 1} gives you the permutation to be performed namely 0↦2, 1↦3, 2↦0, 3↦1.

Now, if you want to consider the signed version what you can do is to replace your graph by a "signed graph" in which the vertices are doubled.

def mat_to_signed_dig(A):
    n = A.nrows()
    G = DiGraph(loops=False, multiedges=False)
    for i in range(n):
        for j in range(n):
            if A[i,j]:
                G.add_edge(i, j, A[i,j])
                G.add_edge(n+i, n+j, A[i,j])
                G.add_edge(i, n+j, -A[i,j])
                G.add_edge(n+i, j, -A[i,j])
    return G

Then

sage: A = matrix(4, [0,1,2,3,4,0,5,6,7,8,0,9,10,11,12,0])
sage: p = matrix(Permutation('(1,3)(2,4)')) * diagonal_matrix([1,-1,-1,1])
sage: B = p * A * ~p
sage: B
[  0  -9  -7   8]
[-12   0  10 -11]
[ -2   3   0  -1]
[  5  -6  -4   0]
sage: G = mat_to_signed_dig(A)
sage: H = mat_to_signed_dig(B)
sage: G.is_isomorphic(H, edge_labels=True, certificate=True)
(True, {0: 6, 1: 3, 2: 0, 3: 5, 4: 2, 5: 7, 6: 4, 7: 1})

The dictionary now corresponds to a signed permutation where i+4 should be thought as -i. So it should be interpreted as the signed permutation +0↦-2, +1↦+3, +2↦+0, +3↦-1, -0↦+2, -1↦-3, -2↦-0, -3->+1.

edit flag offensive delete link more

Comments

Let A and B be two seidel adjacency matrices of simple graphs without loops. The matrices A and B are similar via a signed permutation matrix. Please, how I can find that matrix.??

imy gravatar imageimy ( 2020-10-09 00:36:17 +0100 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

Question Tools

1 follower

Stats

Asked: 2020-02-22 20:44:59 +0100

Seen: 862 times

Last updated: Oct 12 '20