Graded modules with basis#

class sage.categories.graded_modules_with_basis.GradedModulesWithBasis(base_category)#

Bases: GradedModulesCategory

The category of graded modules with a distinguished basis.

EXAMPLES:

sage: C = GradedModulesWithBasis(ZZ); C
Category of graded modules with basis over Integer Ring
sage: sorted(C.super_categories(), key=str)
[Category of filtered modules with basis over Integer Ring,
 Category of graded modules over Integer Ring]
sage: C is ModulesWithBasis(ZZ).Graded()
True
class ElementMethods#

Bases: object

degree_negation()#

Return the image of self under the degree negation automorphism of the graded module to which self belongs.

The degree negation is the module automorphism which scales every homogeneous element of degree \(k\) by \((-1)^k\) (for all \(k\)). This assumes that the module to which self belongs (that is, the module self.parent()) is \(\ZZ\)-graded.

EXAMPLES:

sage: E.<a,b> = ExteriorAlgebra(QQ)                                     # needs sage.modules
sage: ((1 + a) * (1 + b)).degree_negation()                             # needs sage.modules
a*b - a - b + 1
sage: E.zero().degree_negation()                                        # needs sage.modules
0

sage: P = GradedModulesWithBasis(ZZ).example(); P                       # needs sage.combinat sage.modules
An example of a graded module with basis:
 the free module on partitions over Integer Ring
sage: pbp = lambda x: P.basis()[Partition(list(x))]
sage: p = pbp([3,1]) - 2 * pbp([2]) + 4 * pbp([1])                      # needs sage.combinat sage.modules
sage: p.degree_negation()                                               # needs sage.combinat sage.modules
-4*P[1] - 2*P[2] + P[3, 1]
class ParentMethods#

Bases: object

degree_negation(element)#

Return the image of element under the degree negation automorphism of the graded module self.

The degree negation is the module automorphism which scales every homogeneous element of degree \(k\) by \((-1)^k\) (for all \(k\)). This assumes that the module self is \(\ZZ\)-graded.

INPUT:

  • element – element of the module self

EXAMPLES:

sage: E.<a,b> = ExteriorAlgebra(QQ)                                     # needs sage.modules
sage: E.degree_negation((1 + a) * (1 + b))                              # needs sage.modules
a*b - a - b + 1
sage: E.degree_negation(E.zero())                                       # needs sage.modules
0

sage: P = GradedModulesWithBasis(ZZ).example(); P                       # needs sage.combinat sage.modules
An example of a graded module with basis:
 the free module on partitions over Integer Ring
sage: pbp = lambda x: P.basis()[Partition(list(x))]
sage: p = pbp([3,1]) - 2 * pbp([2]) + 4 * pbp([1])                      # needs sage.combinat sage.modules
sage: P.degree_negation(p)                                              # needs sage.combinat sage.modules
-4*P[1] - 2*P[2] + P[3, 1]
quotient_module(submodule, check=True, already_echelonized=False, category=None)#

Construct the quotient module self / submodule.

INPUT:

  • submodule – a submodule with basis of self, or something that can be turned into one via self.submodule(submodule)

  • check, already_echelonized – passed down to ModulesWithBasis.ParentMethods.submodule()

  • category – (optional) the category of the quotient module

Warning

At this point, this only supports quotients by free submodules admitting a basis in unitriangular echelon form. In this case, the quotient is also a free module, with a basis consisting of the retract of a subset of the basis of self.

EXAMPLES:

sage: # needs sage.combinat sage.modules
sage: E.<x,y,z> = ExteriorAlgebra(QQ)
sage: S = E.submodule([x + y, x*y - y*z, y])
sage: Q = E.quotient_module(S)
sage: Q.category()
Join of
 Category of quotients of graded modules with basis over Rational Field and
 Category of graded vector spaces with basis over Rational Field and
 Category of finite dimensional filtered modules with basis over Rational Field

See also

submodule(gens, check=True, already_echelonized=False, unitriangular=False, support_order=None, category=None, *args, **opts)#

Return the submodule spanned by a finite set of elements.

INPUT:

  • gens – a list or family of elements of self

  • check – (default: True) whether to verify that the

    elements of gens are in self

  • already_echelonized – (default: False) whether

    the elements of gens are already in (not necessarily reduced) echelon form

  • unitriangular – (default: False) whether the lift morphism is unitriangular

  • support_order – (optional) either something that can be converted into a tuple or a key function

  • category – (optional) the category of the submodule

If already_echelonized is False, then the generators are put in reduced echelon form using echelonize(), and reindexed by \(0,1,...\).

Warning

At this point, this method only works for finite dimensional submodules and if matrices can be echelonized over the base ring.

If in addition unitriangular is True, then the generators are made such that the coefficients of the pivots are 1, so that lifting map is unitriangular.

The basis of the submodule uses the same index set as the generators, and the lifting map sends \(y_i\) to \(gens[i]\).

See also

EXAMPLES:

A graded submodule of a graded module generated by homogeneous elements is naturally graded:

sage: # needs sage.combinat sage.modules
sage: E.<x,y,z> = ExteriorAlgebra(QQ)
sage: S = E.submodule([x + y, x*y - y*z])
sage: S.category()
Join of
 Category of graded vector spaces with basis over Rational Field and
 Category of subobjects of filtered modules with basis over Rational Field and
 Category of finite dimensional filtered modules with basis over Rational Field
sage: S.basis()[0].degree()
1
sage: S.basis()[1].degree()
2

We check on the echelonized basis:

sage: Sp = E.submodule([1, x + y + 5, x*y - y*z + x + y - 2])           # needs sage.combinat sage.modules
sage: Sp.category()                                                     # needs sage.combinat sage.modules
Join of
 Category of graded vector spaces with basis over Rational Field and
 Category of subobjects of filtered modules with basis over Rational Field and
 Category of finite dimensional filtered modules with basis over Rational Field

If it is generated by inhomogeneous elements, then it is filtered by default:

sage: F = E.submodule([x + y*z, x*z + y*x])                             # needs sage.combinat sage.modules
sage: F.category()                                                      # needs sage.combinat sage.modules
Join of
 Category of subobjects of filtered modules with basis over Rational Field and
 Category of finite dimensional filtered modules with basis over Rational Field and
 Category of filtered vector spaces with basis over Rational Field

If category is specified, then it does not give any extra structure to the submodule (we can think of this as applying the forgetful functor):

sage: # needs sage.combinat sage.modules
sage: SM = E.submodule([x + y, x*y - y*z],
....:                  category=ModulesWithBasis(QQ))
sage: SM.category()
Join of
 Category of finite dimensional vector spaces with basis over Rational Field and
 Category of subobjects of sets
sage: FM = E.submodule([x + 1, x*y - x*y*z],
....:                  category=ModulesWithBasis(QQ))
sage: FM.category()
Join of
 Category of finite dimensional vector spaces with basis over Rational Field and
 Category of subobjects of sets

If we have specified that this is a graded submodule of a graded module, then the echelonized elements must be homogeneous:

sage: Cat = ModulesWithBasis(QQ).Graded().Subobjects()
sage: E.submodule([x + y, x*y - 1], category=Cat)                       # needs sage.combinat sage.modules
Traceback (most recent call last):
...
ValueError: all of the generators must be homogeneous
sage: E.submodule([x + y, x*y - x - y], category=Cat)                   # needs sage.combinat sage.modules
Free module generated by {0, 1} over Rational Field
class Quotients(category, *args)#

Bases: QuotientsCategory

class ElementMethods#

Bases: object

degree()#

Return the degree of self.

EXAMPLES:

sage: # needs sage.combinat sage.modules
sage: E.<x,y,z> = ExteriorAlgebra(QQ)
sage: S = E.submodule([x + y, x*y - y*z, y])
sage: Q = E.quotient_module(S)
sage: B = Q.basis()
sage: [B[i].lift() for i in Q.indices()]
[1, z, x*z, y*z, x*y*z]
sage: [B[i].degree() for i in Q.indices()]
[0, 1, 2, 2, 3]
class ParentMethods#

Bases: object

degree_on_basis(m)#

Return the degree of the basis element indexed by m in self.

EXAMPLES:

sage: # needs sage.combinat sage.modules
sage: E.<x,y,z> = ExteriorAlgebra(QQ)
sage: S = E.submodule([x + y, x*y - y*z, y])
sage: Q = E.quotient_module(S)
sage: B = Q.basis()
sage: [B[i].lift() for i in Q.indices()]
[1, z, x*z, y*z, x*y*z]
sage: [Q.degree_on_basis(i) for i in Q.indices()]
[0, 1, 2, 2, 3]