# Free quadratic modules#

Sage supports computation with free quadratic modules over an arbitrary commutative ring. Nontrivial functionality is available over $$\ZZ$$ and fields. All free modules over an integral domain are equipped with an embedding in an ambient vector space and an inner product, which you can specify and change.

Create the free module of rank $$n$$ over an arbitrary commutative ring $$R$$ using the command FreeModule(R,n) with a given inner_product_matrix.

The following example illustrates the creation of both a vector space and a free module over the integers and a submodule of it. Use the functions FreeModule, span and member functions of free modules to create free modules. ‘’Do not use the FreeModule_xxx constructors directly.’’

EXAMPLES:

sage: M = Matrix(QQ,[[2,1,0],[1,2,1],[0,1,2]])
sage: V = VectorSpace(QQ,3,inner_product_matrix=M)
sage: type(V)
sage: V.inner_product_matrix()
[2 1 0]
[1 2 1]
[0 1 2]
sage: W = V.subspace([[1,2,7], [1,1,0]])
sage: type(W)
sage: W
Quadratic space of degree 3 and dimension 2 over Rational Field
Basis matrix:
[ 1  0 -7]
[ 0  1  7]
Inner product matrix:
[2 1 0]
[1 2 1]
[0 1 2]
sage: W.gram_matrix()
[ 100 -104]
[-104  114]


AUTHORS:

• David Kohel (2008-06): First created (based on free_module.py)

Create the free quadratic module over the given commutative ring of the given rank.

INPUT:

• base_ring – a commutative ring

• rank – a nonnegative integer

• inner_product_matrix – the inner product matrix

• sparse – bool; (default False)

• inner_product_ring – the inner product codomain ring; (default None)

OUTPUT:

A free quadratic module (with given inner product matrix).

Note

In Sage, it is the case that there is only one dense and one sparse free ambient quadratic module of rank $$n$$ over $$R$$ and given inner product matrix.

EXAMPLES:

sage: M2 = FreeQuadraticModule(ZZ,2,inner_product_matrix=[1,2,3,4])
sage: M2 is FreeQuadraticModule(ZZ,2,inner_product_matrix=[1,2,3,4])
True
sage: M2.inner_product_matrix()
[1 2]
[3 4]
sage: M3 = FreeModule(ZZ,2,inner_product_matrix=[[1,2],[3,4]])
sage: M3 is M2
True


Ambient free module over a commutative ring.

Ambient free quadratic module over an integral domain.

ambient_vector_space()#

Return the ambient vector space, which is this free module tensored with its fraction field.

EXAMPLES:

sage: M = ZZ^3;  M.ambient_vector_space()
Vector space of dimension 3 over Rational Field


Create the ambient vector space of given dimension over the given field.

INPUT:

• base_field – a field

• dimension – a non-negative integer

• sparse – bool (default: False)

EXAMPLES:

sage: VectorSpace(QQ,3,inner_product_matrix=[[2,1,0],[1,2,0],[0,1,2]])
Ambient quadratic space of dimension 3 over Rational Field
Inner product matrix:
[2 1 0]
[1 2 0]
[0 1 2]


Ambient free quadratic module over a principal ideal domain.

Base class for all free quadratic modules.

Modules are ordered by inclusion in the same ambient space.

ambient_module()#

Return the ambient module associated to this module.

EXAMPLES:

sage: R.<x,y> = QQ[]
sage: M = FreeModule(R,2)
sage: M.ambient_module()
Ambient free module of rank 2 over the integral domain Multivariate Polynomial Ring in x, y over Rational Field

sage: V = FreeModule(QQ, 4).span([[1,2,3,4], [1,0,0,0]]); V
Vector space of degree 4 and dimension 2 over Rational Field
Basis matrix:
[  1   0   0   0]
[  0   1 3/2   2]
sage: V.ambient_module()
Vector space of dimension 4 over Rational Field

determinant()#

Return the determinant of this free module.

EXAMPLES:

sage: M = FreeModule(ZZ, 3, inner_product_matrix=1)
sage: M.determinant()
1
sage: N = M.span([[1,2,3]])
sage: N.determinant()
14
sage: P = M.span([[1,2,3], [1,1,1]])
sage: P.determinant()
6

discriminant()#

Return the discriminant of this free module.

This is defined to be $$(-1)^r$$ of the determinant, where $$r = n/2$$ ($$n$$ even) or $$(n-1)/2$$ ($$n$$ odd) for a module of rank $$n$$.

EXAMPLES:

sage: M = FreeModule(ZZ, 3)
sage: M.discriminant()
1
sage: N = M.span([[1,2,3]])
sage: N.discriminant()
14
sage: P = M.span([[1,2,3], [1,1,1]])
sage: P.discriminant()
6

gram_matrix()#

Return the Gram matrix associated to this free module.

This is defined to be B*A*B.transpose(), where A is the inner product matrix (induced from the ambient space), and B the basis matrix.

EXAMPLES:

sage: V = VectorSpace(QQ,4)
sage: u = V([1/2,1/2,1/2,1/2])
sage: v = V([0,1,1,0])
sage: w = V([0,0,1,1])
sage: M = span([u,v,w], ZZ)
sage: M.inner_product_matrix() == V.inner_product_matrix()
True
sage: L = M.submodule_with_basis([u,v,w])
sage: L.inner_product_matrix() == M.inner_product_matrix()
True
sage: L.gram_matrix()
[1 1 1]
[1 2 1]
[1 1 2]

inner_product_matrix()#

Return the inner product matrix associated to this module.

By definition, this is the inner product matrix of the ambient space, hence may be of degree greater than the rank of the module.

Note

The inner product does not have to be symmetric (see examples).

Todo

Differentiate the image ring of the inner product from the base ring of the module and/or ambient space. E.g. On an integral module over ZZ the inner product pairing could naturally take values in ZZ, QQ, RR, or CC.

EXAMPLES:

sage: M = FreeModule(ZZ, 3)
sage: M.inner_product_matrix()
[1 0 0]
[0 1 0]
[0 0 1]


The inner product does not have to be symmetric or definite:

sage: N = FreeModule(ZZ,2,inner_product_matrix=[[1,-1],[2,5]])
sage: N.inner_product_matrix()
[ 1 -1]
[ 2  5]
sage: u, v = N.basis()
sage: u.inner_product(v)
-1
sage: v.inner_product(u)
2


The inner product matrix is defined with respect to the ambient space:

sage: V = QQ^3
sage: u = V([1/2,1,1])
sage: v = V([1,1,1/2])
sage: M = span([u,v], ZZ)
sage: M.inner_product_matrix()
[1 0 0]
[0 1 0]
[0 0 1]
sage: M.inner_product_matrix() == V.inner_product_matrix()
True
sage: M.gram_matrix()
[ 1/2 -3/4]
[-3/4 13/4]


Base class for all free modules over fields.

Return the $$K$$-span of the given list of gens, where $$K$$ is the base field of self.

Note that this span is a subspace of the ambient vector space, but need not be a subspace of self.

INPUT:

• gens – list of vectors

• check – bool (default: True): whether or not to coerce entries of gens into base field

• already_echelonized – bool (default: False): set this if you know the gens are already in echelon form

EXAMPLES:

sage: V = VectorSpace(GF(7), 3)
sage: W = V.subspace([[2,3,4]]); W
Vector space of degree 3 and dimension 1 over Finite Field of size 7
Basis matrix:
[1 5 2]
sage: W.span([[1,1,1]])
Vector space of degree 3 and dimension 1 over Finite Field of size 7
Basis matrix:
[1 1 1]


Return the free $$K$$-module with the given basis, where $$K$$ is the base field of self.

Note that this span is a subspace of the ambient vector space, but need not be a subspace of self.

INPUT:

• basis – list of vectors

• check – bool (default: True): whether or not to coerce entries of gens into base field

• already_echelonized – bool (default: False): set this if you know the gens are already in echelon form

EXAMPLES:

sage: V = VectorSpace(GF(7), 3)
sage: W = V.subspace([[2,3,4]]); W
Vector space of degree 3 and dimension 1 over Finite Field of size 7
Basis matrix:
[1 5 2]
sage: W.span_of_basis([[2,2,2], [3,3,0]])
Vector space of degree 3 and dimension 2 over Finite Field of size 7
User basis matrix:
[2 2 2]
[3 3 0]


The basis vectors must be linearly independent or a ValueError exception is raised:

sage: W.span_of_basis([[2,2,2], [3,3,3]])
Traceback (most recent call last):
...
ValueError: The given basis vectors must be linearly independent.


Class of all free modules over a PID.

Return the $$R$$-span of the given list of gens, where $$R$$ is the base ring of self.

Note that this span need not be a submodule of self, nor even of the ambient space. It must, however, be contained in the ambient vector space, i.e., the ambient space tensored with the fraction field of $$R$$.

EXAMPLES:

sage: V = FreeModule(ZZ,3)
sage: W = V.submodule([V.gen(0)])
sage: W.span([V.gen(1)])
Free module of degree 3 and rank 1 over Integer Ring
Echelon basis matrix:
[0 1 0]
sage: W.submodule([V.gen(1)])
Traceback (most recent call last):
...
ArithmeticError: argument gens (= [(0, 1, 0)]) does not generate a submodule of self


Return the free $$R$$-module with the given basis, where $$R$$ is the base ring of self.

Note that this $$R$$-module need not be a submodule of self, nor even of the ambient space. It must, however, be contained in the ambient vector space, i.e., the ambient space tensored with the fraction field of $$R$$.

EXAMPLES:

sage: M = FreeModule(ZZ,3)
sage: W = M.span_of_basis([M([1,2,3])])


Next we create two free $$\ZZ$$-modules, neither of which is a submodule of $$W$$:

sage: W.span_of_basis([M([2,4,0])])
Free module of degree 3 and rank 1 over Integer Ring
User basis matrix:
[2 4 0]


The following module is not even in the ambient space:

sage: Q = QQ
sage: W.span_of_basis([ Q('1/5')*M([1,2,0]), Q('1/7')*M([1,1,0]) ])
Free module of degree 3 and rank 2 over Integer Ring
User basis matrix:
[1/5 2/5   0]
[1/7 1/7   0]


Of course the input basis vectors must be linearly independent:

sage: W.span_of_basis([ [1,2,0], [2,4,0] ])
Traceback (most recent call last):
...
ValueError: The given basis vectors must be linearly independent.

zero_submodule()#

Return the zero submodule of this module.

EXAMPLES:

sage: V = FreeModule(ZZ,2)
sage: V.zero_submodule()
Free module of degree 2 and rank 0 over Integer Ring
Echelon basis matrix:
[]


An embedded vector subspace with echelonized basis.

EXAMPLES:

Since this is an embedded vector subspace with echelonized basis, the methods echelon_coordinates() and coordinates() return the same coordinates:

sage: V = QQ^3
sage: W = V.span([[1,2,3],[4,5,6]])
sage: W
Vector space of degree 3 and dimension 2 over Rational Field
Basis matrix:
[ 1  0 -1]
[ 0  1  2]

sage: v = V([1,5,9])
sage: W.echelon_coordinates(v)
[1, 5]
sage: vector(QQ, W.echelon_coordinates(v)) * W.basis_matrix()
(1, 5, 9)

sage: v = V([1,5,9])
sage: W.coordinates(v)
[1, 5]
sage: vector(QQ, W.coordinates(v)) * W.basis_matrix()
(1, 5, 9)


An $$R$$-submodule of $$K^n$$ where $$K$$ is the fraction field of a principal ideal domain $$R$$.

EXAMPLES:

sage: M = ZZ^3
sage: W = M.span_of_basis([[1,2,3],[4,5,19]]); W
Free module of degree 3 and rank 2 over Integer Ring
User basis matrix:
[ 1  2  3]
[ 4  5 19]


We can save and load submodules and elements:

sage: loads(W.dumps()) == W
True
sage: v = W.0 + W.1
sage: loads(v.dumps()) == v
True


An embedded vector subspace with a distinguished user basis.

EXAMPLES:

sage: M = QQ^3; W = M.submodule_with_basis([[1,2,3], [4,5,19]]); W
Vector space of degree 3 and dimension 2 over Rational Field
User basis matrix:
[ 1  2  3]
[ 4  5 19]


Since this is an embedded vector subspace with a distinguished user basis possibly different than the echelonized basis, the echelon_coordinates() and user coordinates() do not agree:

sage: V = QQ^3
sage: W = V.submodule_with_basis([[1,2,3], [4,5,6]])
sage: W
Vector space of degree 3 and dimension 2 over Rational Field
User basis matrix:
[1 2 3]
[4 5 6]

sage: v = V([1,5,9])
sage: W.echelon_coordinates(v)
[1, 5]
sage: vector(QQ, W.echelon_coordinates(v)) * W.echelonized_basis_matrix()
(1, 5, 9)

sage: v = V([1,5,9])
sage: W.coordinates(v)
[5, -1]
sage: vector(QQ, W.coordinates(v)) * W.basis_matrix()
(1, 5, 9)


We can load and save submodules:

sage: loads(W.dumps()) == W
True

sage: K.<x> = FractionField(PolynomialRing(QQ,'x'))
sage: M = K^3; W = M.span_of_basis([[1,1,x]])
sage: loads(W.dumps()) == W
True


An $$R$$-submodule of $$K^n$$ with distinguished basis, where $$K$$ is the fraction field of a principal ideal domain $$R$$.

Modules are ordered by inclusion.

EXAMPLES:

First we compare two equal vector spaces:

sage: A = FreeQuadraticModule(QQ,3,2*matrix.identity(3))
sage: V = A.span([[1,2,3], [5,6,7], [8,9,10]])
sage: W = A.span([[5,6,7], [8,9,10]])
sage: V == W
True


Next we compare a one dimensional space to the two dimensional space defined above:

sage: M = A.span([[5,6,7]])
sage: V == M
False
sage: M < V
True
sage: V < M
False


We compare a $$\ZZ$$-module to the one-dimensional space above:

sage: V = A.span([[5,6,7]])
sage: V = V.change_ring(ZZ).scale(1/11)
sage: V < M
True
sage: M < V
False

change_ring(R)#

Return the free module over $$R$$ obtained by coercing each element of self into a vector over the fraction field of $$R$$, then taking the resulting $$R$$-module.

This raises a TypeError if coercion is not possible.

INPUT:

• R – a principal ideal domain

EXAMPLES:

Changing rings preserves the inner product and the user basis:

sage: V = QQ^3
sage: W = V.subspace([[2, '1/2', 1]]); W
Vector space of degree 3 and dimension 1 over Rational Field
Basis matrix:
[  1 1/4 1/2]
sage: W.change_ring(GF(7))
Vector space of degree 3 and dimension 1 over Finite Field of size 7
Basis matrix:
[1 2 4]

sage: N = FreeModule(ZZ, 2, inner_product_matrix=[[1,-1],[2,5]])
sage: N.inner_product_matrix()
[ 1 -1]
[ 2  5]
sage: Np = N.change_ring(RDF)
sage: Np.inner_product_matrix()
[ 1.0 -1.0]
[ 2.0  5.0]

sage.modules.free_quadratic_module.InnerProductSpace(K, dimension, inner_product_matrix, sparse=False)#

EXAMPLES:

The base can be complicated, as long as it is a field:

sage: F.<x> = FractionField(PolynomialRing(ZZ,'x'))
sage: D = diagonal_matrix([x,x-1,x+1])
sage: V = QuadraticSpace(F,3,D)
sage: V
Ambient quadratic space of dimension 3 over Fraction Field of Univariate Polynomial Ring in x over Integer Ring
Inner product matrix:
[    x     0     0]
[    0 x - 1     0]
[    0     0 x + 1]
sage: V.basis()
[
(1, 0, 0),
(0, 1, 0),
(0, 0, 1)
]


The base must be a field or a TypeError is raised:

sage: QuadraticSpace(ZZ,5,identity_matrix(ZZ,2))
Traceback (most recent call last):
...
TypeError: argument K (= Integer Ring) must be a field


EXAMPLES:

The base can be complicated, as long as it is a field:

sage: F.<x> = FractionField(PolynomialRing(ZZ,'x'))
sage: D = diagonal_matrix([x,x-1,x+1])
sage: V = QuadraticSpace(F,3,D)
sage: V
Ambient quadratic space of dimension 3 over Fraction Field of Univariate Polynomial Ring in x over Integer Ring
Inner product matrix:
[    x     0     0]
[    0 x - 1     0]
[    0     0 x + 1]
sage: V.basis()
[
(1, 0, 0),
(0, 1, 0),
(0, 0, 1)
]


The base must be a field or a TypeError is raised:

sage: QuadraticSpace(ZZ,5,identity_matrix(ZZ,2))
Traceback (most recent call last):
...
TypeError: argument K (= Integer Ring) must be a field


Return True if $$M$$ is a free quadratic module.

EXAMPLES:

sage: from sage.modules.free_quadratic_module import is_FreeQuadraticModule
sage: U = FreeModule(QQ,3)