def p_adic_normal_form(G, p, precision=None, partial=False, debug=False):
r"""
Return the transformation to the `p`-adic normal form of a symmetric matrix.
Two ``p-adic`` quadratic forms are integrally equivalent if and only if
their Gram matrices have the same normal form.
Let `p` be odd and `u` be the smallest non-square modulo `p`.
The normal form is a block diagonal matrix
with blocks `p^k G_k` such that `G_k` is either the identity matrix or
the identity matrix with the last diagonal entry replaced by `u`.
If `p=2` is even, define the `1` by `1` matrices::
sage: W1 = Matrix([1]); W1
[1]
sage: W3 = Matrix([3]); W3
[3]
sage: W5 = Matrix([5]); W5
[5]
sage: W7 = Matrix([7]); W7
[7]
and the `2` by `2` matrices::
sage: U = Matrix(2,[0,1,1,0]); U
[0 1]
[1 0]
sage: V = Matrix(2,[2,1,1,2]); V
[2 1]
[1 2]
For `p=2` the partial normal form is a block diagonal matrix with blocks
`2^k G_k` such that $G_k$ is a block diagonal matrix of the form
`[U`, ... , `U`, `V`, `Wa`, `Wb]`
where we allow `V`, `Wa`, `Wb` to be `0 \times 0` matrices.
Further restrictions to the full normal form apply.
We refer to [MirMor2009]_ IV Definition 4.6. for the details.
INPUT:
- ``G`` -- a symmetric `n` by `n` matrix in `\QQ`
- ``p`` -- a prime number -- it is not checked whether it is prime
- ``precision`` -- if not set, the minimal possible is taken
- ``partial`` -- boolean (default: ``False``) if set, only the
partial normal form is returned.
OUTPUT:
- ``D`` -- the jordan matrix over `\QQ_p`
- ``B`` -- invertible transformation matrix over `\ZZ_p`,
i.e, ``D = B * G * B^T``
EXAMPLES::
sage: from sage.quadratic_forms.genera.normal_form import p_adic_normal_form
sage: D4 = Matrix(ZZ, 4, [2,-1,-1,-1,-1,2,0,0,-1,0,2,0,-1,0,0,2])
sage: D4
[ 2 -1 -1 -1]
[-1 2 0 0]
[-1 0 2 0]
[-1 0 0 2]
sage: D, B = p_adic_normal_form(D4, 2)
sage: D
[ 2 1 0 0]
[ 1 2 0 0]
[ 0 0 2^2 2]
[ 0 0 2 2^2]
sage: D == B * D4 * B.T
True
sage: A4 = Matrix(ZZ, 4, [2, -1, 0, 0, -1, 2, -1, 0, 0, -1, 2, -1, 0, 0, -1, 2])
sage: A4
[ 2 -1 0 0]
[-1 2 -1 0]
[ 0 -1 2 -1]
[ 0 0 -1 2]
sage: D, B = p_adic_normal_form(A4, 2)
sage: D
[0 1 0 0]
[1 0 0 0]
[0 0 2 1]
[0 0 1 2]
We can handle degenerate forms::
sage: A4_extended = Matrix(ZZ, 5, [2, -1, 0, 0, -1, -1, 2, -1, 0, 0, 0, -1, 2, -1, 0, 0, 0, -1, 2, -1, -1, 0, 0, -1, 2])
sage: D, B = p_adic_normal_form(A4_extended, 5)
sage: D
[1 0 0 0 0]
[0 1 0 0 0]
[0 0 1 0 0]
[0 0 0 5 0]
[0 0 0 0 0]
and denominators::
sage: A4dual = A4.inverse()
sage: D, B = p_adic_normal_form(A4dual, 5)
sage: D
[5^-1 0 0 0]
[ 0 1 0 0]
[ 0 0 1 0]
[ 0 0 0 1]
TESTS::
sage: Z = Matrix(ZZ,0,[])
sage: p_adic_normal_form(Z, 3)
([], [])
sage: Z = matrix.zero(10)
sage: p_adic_normal_form(Z, 3)[0] == 0
True
"""
p = ZZ(p)
# input checks!!
G0, denom = G._clear_denom()
d = denom.valuation(p)
r = G0.rank()
if r != G0.ncols():
U = G0.hermite_form(transformation=True)[1]
else:
U = G0.parent().identity_matrix()
kernel = U[r:, :]
nondeg = U[:r, :]
# continue with the non-degenerate part
G = nondeg * G * nondeg.T * p**d
if precision is None:
# in Zp(2) we have to calculate at least mod 8 for things to make sense.
precision = G.det().valuation(p) + 4
R = Zp(p, prec=precision, type='fixed-mod')
G = G.change_ring(R)
G.set_immutable() # is not changed during computation
D = copy(G) # is transformed into jordan form
n = G.ncols()
# The trivial case
if n == 0:
return G.parent().zero(), G.parent().zero()
# the transformation matrix is called B
B = Matrix.identity(R, n)
if p == 2:
D, B = _jordan_2_adic(G)
else:
D, B = _jordan_odd_adic(G)
D, B1 = _normalize(D)
B = B1 * B
# we have reached a normal form for p != 2
# for p == 2 extra work is necessary
if p == 2:
D, B1 = _two_adic_normal_forms(D, partial=partial)
B = B1 * B
nondeg = B * nondeg
B = nondeg.stack(kernel)
D = Matrix.block_diagonal([D, Matrix.zero(kernel.nrows())])
if debug:
assert B.determinant().valuation() == 0 # B is invertible!
if p == 2:
assert B * G * B.T == Matrix.block_diagonal(
collect_small_blocks(D))
else:
assert B * G * B.T == Matrix.diagonal(D.diagonal())
return D / p**d, B
def normal_form(self, partial=False):
r"""
Return the normal form of this torsion quadratic module.
Two torsion quadratic modules are isomorphic if and only if they have
the same value modules and the same normal form.
A torsion quadratic module `(T,q)` with values in `\QQ/n\ZZ` is
in normal form if the rescaled quadratic module `(T, q/n)`
with values in `\QQ/\ZZ` is in normal form.
For the definition of normal form see [MirMor2009]_ IV Definition 4.6.
Below are some of its properties.
Let `p` be odd and `u` be the smallest non-square modulo `p`.
The normal form is a diagonal matrix with diagonal entries either `p^n`
or `u p^n`.
If `p = 2` is even, then the normal form consists of
1 x 1 blocks of the form
.. MATH::
(0), \quad 2^n(1),\quad 2^n(3),\quad 2^n(5) ,\quad 2^n(7)
or of `2 \times 2` blocks of the form
.. MATH::
2^n
\left(\begin{matrix}
2 & 1\\
1 & 2
\end{matrix}\right), \quad
2^n
\left(\begin{matrix}
0 & 1\\
1 & 0
\end{matrix}\right).
The blocks are ordered by their valuation.
INPUT:
- partial - bool (default: ``False``) return only a partial normal form
it is not unique but still useful to extract invariants
OUTPUT:
- a torsion quadratic module
EXAMPLES::
sage: L1=IntegralLattice(matrix([[-2,0,0],[0,1,0],[0,0,4]]))
sage: L1.discriminant_group().normal_form()
Finite quadratic module over Integer Ring with invariants (2, 4)
Gram matrix of the quadratic form with values in Q/Z:
[1/2 0]
[ 0 1/4]
sage: L2=IntegralLattice(matrix([[-2,0,0],[0,1,0],[0,0,-4]]))
sage: L2.discriminant_group().normal_form()
Finite quadratic module over Integer Ring with invariants (2, 4)
Gram matrix of the quadratic form with values in Q/Z:
[1/2 0]
[ 0 1/4]
We check that :trac:`24864` is fixed::
sage: L1=IntegralLattice(matrix([[-4,0,0],[0,4,0],[0,0,-2]]))
sage: AL1=L1.discriminant_group()
sage: L2=IntegralLattice(matrix([[-4,0,0],[0,-4,0],[0,0,2]]))
sage: AL2=L2.discriminant_group()
sage: AL1.normal_form()
Finite quadratic module over Integer Ring with invariants (2, 4, 4)
Gram matrix of the quadratic form with values in Q/2Z:
[1/2 0 0]
[ 0 1/4 0]
[ 0 0 5/4]
sage: AL2.normal_form()
Finite quadratic module over Integer Ring with invariants (2, 4, 4)
Gram matrix of the quadratic form with values in Q/2Z:
[1/2 0 0]
[ 0 1/4 0]
[ 0 0 5/4]
Some exotic cases::
sage: from sage.modules.torsion_quadratic_module import TorsionQuadraticModule
sage: D4_gram = Matrix(ZZ,4,4,[2,0,0,-1,0,2,0,-1,0,0,2,-1,-1,-1,-1,2])
sage: D4 = FreeQuadraticModule(ZZ,4,D4_gram)
sage: D4dual = D4.span(D4_gram.inverse())
sage: T = TorsionQuadraticModule((1/6)*D4dual,D4)
sage: T
Finite quadratic module over Integer Ring with invariants (6, 6, 12, 12)
Gram matrix of the quadratic form with values in Q/(1/3)Z:
[ 1/18 1/12 5/36 1/36]
[ 1/12 1/6 1/36 1/9]
[ 5/36 1/36 1/36 11/72]
[ 1/36 1/9 11/72 1/36]
sage: T.normal_form()
Finite quadratic module over Integer Ring with invariants (6, 6, 12, 12)
Gram matrix of the quadratic form with values in Q/(1/3)Z:
[ 1/6 1/12 0 0 0 0 0 0]
[1/12 1/6 0 0 0 0 0 0]
[ 0 0 1/12 1/24 0 0 0 0]
[ 0 0 1/24 1/12 0 0 0 0]
[ 0 0 0 0 1/9 0 0 0]
[ 0 0 0 0 0 1/9 0 0]
[ 0 0 0 0 0 0 1/9 0]
[ 0 0 0 0 0 0 0 1/9]
TESTS:
A degenerate case::
sage: T = TorsionQuadraticModule((1/6)*D4dual, D4, modulus=1/36)
sage: T.normal_form()
Finite quadratic module over Integer Ring with invariants (6, 6, 12, 12)
Gram matrix of the quadratic form with values in Q/(1/18)Z:
[1/36 1/72 0 0 0 0 0 0]
[1/72 1/36 0 0 0 0 0 0]
[ 0 0 0 0 0 0 0 0]
[ 0 0 0 0 0 0 0 0]
[ 0 0 0 0 0 0 0 0]
[ 0 0 0 0 0 0 0 0]
[ 0 0 0 0 0 0 0 0]
[ 0 0 0 0 0 0 0 0]
"""
gens = []
from sage.quadratic_forms.genera.normal_form import p_adic_normal_form, _normalize
for p in self.annihilator().gen().prime_divisors():
D_p = self.primary_part(p)
q_p = D_p.gram_matrix_quadratic()
q_p = q_p / D_p._modulus_qf
# continue with the non-degenerate part
r = q_p.rank()
if r != q_p.ncols():
U = q_p._clear_denom()[0].hermite_form(transformation=True)[1]
else:
U = q_p.parent().identity_matrix()
kernel = U[r:, :]
nondeg = U[:r, :]
q_p = nondeg * q_p * nondeg.T
# the normal form is implemented for p-adic lattices
# so we should work with the lattice q_p --> q_p^-1
q_p1 = q_p.inverse()
prec = self.annihilator().gen().valuation(p) + 5
D, U = p_adic_normal_form(q_p1, p, precision=prec + 5, partial=partial)
# if we compute the inverse in the p-adics everything explodes --> go to ZZ
U = U.change_ring(ZZ).inverse().transpose()
# the inverse is in normal form - so to get a normal form for the original one
# it is enough to massage each 1x1 resp. 2x2 block.
U = U.change_ring(Zp(p, type='fixed-mod', prec=prec)).change_ring(ZZ)
D = U * q_p * U.T * p**q_p.denominator().valuation(p)
D = D.change_ring(Zp(p, type='fixed-mod', prec=prec))
_, U1 = _normalize(D, normal_odd=False)
U = U1.change_ring(ZZ) * U
# reattach the degenerate part
nondeg = U * nondeg
U = nondeg.stack(kernel)
# apply U to the generators
n = U.ncols()
gens_p = []
for i in range(n):
g = self.V().zero()
for j in range(n):
g += D_p.gens()[j].lift() * U[i, j]
gens_p.append(g)
gens += gens_p
return self.submodule_with_gens(gens)
