ASKSAGE: Sage Q&A Forum - RSS feedhttps://ask.sagemath.org/questions/Q&A Forum for SageenCopyright Sage, 2010. Some rights reserved under creative commons license.Wed, 01 Mar 2017 23:52:53 +0100Unitary matrices over finite fieldshttps://ask.sagemath.org/question/9667/unitary-matrices-over-finite-fields/Hi everyone
I was delighted to see that the SAGE team had implemented the unitary groups GU(n,q) and SU(n,q), since they are such peculiar objects; however the implementation does not seem to include many matrix functions (eg det, trace, transpose etc.).
Let S be a (small) subset of GF(q^2). Let G=GU(n,q) and define a subset D_S of G (ie D_S is NOT a vector subspace or subgroup or anything) to be the matrices whose entries may only be chosen from this particular subset S. Let U, V in D_S. I need to check the matrix products (U^t)V for membership of D_S, where U^t denotes U.transpose() acted on by Frobenius in the usual way, and ideally I would like to store the set of pairs P = {(U,V) satisfying (U^t)V in D_S}, or even better to store some "generators" for P.
I have two questions:
(1) which matrix operations are available for elements of GU(n,q)?
(2) is there an easy way of specifying a sub-SET like D_S so that such a search may be performed? Needless to say once q and/or n gets any bigger than 3, holding any of these subsets naively in memory becomes prohibitively costly!
Many thanks for any help.
PS I can find generators for these groups using G.gens(); how please do I find relations between the gens?Sun, 30 Dec 2012 12:58:09 +0100https://ask.sagemath.org/question/9667/unitary-matrices-over-finite-fields/Answer by dan_fulea for <p>Hi everyone</p>
<p>I was delighted to see that the SAGE team had implemented the unitary groups GU(n,q) and SU(n,q), since they are such peculiar objects; however the implementation does not seem to include many matrix functions (eg det, trace, transpose etc.).</p>
<p>Let S be a (small) subset of GF(q^2). Let G=GU(n,q) and define a subset D_S of G (ie D_S is NOT a vector subspace or subgroup or anything) to be the matrices whose entries may only be chosen from this particular subset S. Let U, V in D_S. I need to check the matrix products (U^t)V for membership of D_S, where U^t denotes U.transpose() acted on by Frobenius in the usual way, and ideally I would like to store the set of pairs P = {(U,V) satisfying (U^t)V in D_S}, or even better to store some "generators" for P.</p>
<p>I have two questions: </p>
<p>(1) which matrix operations are available for elements of GU(n,q)?</p>
<p>(2) is there an easy way of specifying a sub-SET like D_S so that such a search may be performed? Needless to say once q and/or n gets any bigger than 3, holding any of these subsets naively in memory becomes prohibitively costly!</p>
<p>Many thanks for any help.</p>
<p>PS I can find generators for these groups using G.gens(); how please do I find relations between the gens?</p>
https://ask.sagemath.org/question/9667/unitary-matrices-over-finite-fields/?answer=36781#post-id-36781
The setting is clear, but the question somehow interferes with the mathematical research question of finding a good $S$, subset in $\mathbb{F}_{p^2}$, $p$ prime.
My structural problem is that $S$ should be good for all entries in a matrix to be validated.
The property is defined elementwise, so we need the matrix with its entries.
(Some triangular or "parabolic" shapes are eliminated.)
Now to the questions:
(1) which matrix operations are available for elements of GU(n,q)?
I tried:
sage: G = GU( 3, 7 )
sage: G
General Unitary Group of degree 3 over Finite Field in a of size 7^2
sage: G.order().factor()
2^10 * 3 * 7^3 * 43
sage: A = G.random_element()
sage: A
[ a + 1 5 4*a + 1]
[ 6*a 6*a + 3 6*a]
[5*a + 6 2*a + 1 3*a + 1]
sage: A.
A.N A.conjugacy_class A.inverse A.matrix A.parent A.save
A.base_extend A.db A.is_idempotent A.multiplicative_order A.powers A.subs
A.base_ring A.dump A.is_one A.n A.pseudo_order A.substitute
A.cartesian_product A.dumps A.is_zero A.numerical_approx A.rename A.word_problem
A.category A.gap A.list A.order A.reset_name
Above, after A. i was hitting the TAB (twice), and the list of implemented (public) methods was printed.
An other way of investigating A is by printing dir(A).
The "hidden" methods show themselves after A._TABULATOR . (In the interpreter.)
The we can use for instance the method order.
sage: A.order().factor()
2^3 * 43
Yes, determinant is not in the landscape. I tried to coerce, to apply a morphism.
Finally, the following work around could indeed walk around.
p = 7
G = GU( 3, p )
F.<a> = GF( p^2 )
H = GL( 3, F )
H_Matrices = [] # and we will append.
for A in G.gens():
HA = H ( A.gap().sage() ) # tricky coercion, not proud of it
H_Matrices.append( HA )
HG = H.subgroup( H_Matrices )
print "HG has order: %s" % HG.order().factor()
And we get:
HG has order: 2^10 * 3 * 7^3 * 43
And in HG we also have the determinant. For instance:
h = HG.random_element()
print "The matrix\n%s\nhas determinant %s" % ( h, h.matrix().det() )
This gives a value in $\pm 1$:
The matrix
[ a 6*a + 1 4*a + 1]
[5*a + 5 3*a 2*a + 2]
[2*a + 3 3*a + 3 a + 3]
has determinant 6
(2)
For the second question, it is easy to implement a validator.
For instance:
A, B = H_Matrices
A = A.matrix()
B = B.matrix()
S = set( A.list() + B.list() )
# S is the list of entries in A, B
FA = H( [ F.frobenius_endomorphism()( entry ) for entry in A.list() ] ).matrix()
FB = H( [ F.frobenius_endomorphism()( entry ) for entry in B.list() ] ).matrix()
So we obtain the $F'$ action on matrices, which invokes the Frobenius $F$ and the reversion of rows and columns,
that invariates $A$.
Now we can build products, and check if the entries land in S.
Wed, 01 Mar 2017 23:52:53 +0100https://ask.sagemath.org/question/9667/unitary-matrices-over-finite-fields/?answer=36781#post-id-36781