# Filtered Algebras With Basis¶

A filtered algebra with basis over a commutative ring $$R$$ is a filtered algebra over $$R$$ endowed with the structure of a filtered module with basis (with the same underlying filtered-module structure). See FilteredAlgebras and FilteredModulesWithBasis for these two notions.

class sage.categories.filtered_algebras_with_basis.FilteredAlgebrasWithBasis(base_category)

The category of filtered algebras with a distinguished homogeneous basis.

A filtered algebra with basis over a commutative ring $$R$$ is a filtered algebra over $$R$$ endowed with the structure of a filtered module with basis (with the same underlying filtered-module structure). See FilteredAlgebras and FilteredModulesWithBasis for these two notions.

EXAMPLES:

sage: C = AlgebrasWithBasis(ZZ).Filtered(); C
Category of filtered algebras with basis over Integer Ring
sage: sorted(C.super_categories(), key=str)
[Category of algebras with basis over Integer Ring,
Category of filtered algebras over Integer Ring,
Category of filtered modules with basis over Integer Ring]

class ElementMethods

Bases: object

class ParentMethods

Bases: object

from_graded_conversion()

Return the inverse of the canonical $$R$$-module isomorphism $$A \to \operatorname{gr} A$$ induced by the basis of $$A$$ (where $$A =$$). This inverse is an isomorphism $$\operatorname{gr} A \to A$$.

This is an isomorphism of $$R$$-modules, not of algebras. See the class documentation AssociatedGradedAlgebra.

EXAMPLES:

sage: A = Algebras(QQ).WithBasis().Filtered().example()
sage: p = A.an_element() + A.algebra_generators()['x'] + 2; p
U['x']^2*U['y']^2*U['z']^3 + 3*U['x'] + 3*U['y'] + 3
True
True

graded_algebra()

Return the associated graded algebra to self.

See AssociatedGradedAlgebra for the definition and the properties of this.

If the filtered algebra self with basis is called $$A$$, then this method returns $$\operatorname{gr} A$$. The method to_graded_conversion() returns the canonical $$R$$-module isomorphism $$A \to \operatorname{gr} A$$ induced by the basis of $$A$$, and the method from_graded_conversion() returns the inverse of this isomorphism. The method projection() projects elements of $$A$$ onto $$\operatorname{gr} A$$ according to their place in the filtration on $$A$$.

Warning

When not overridden, this method returns the default implementation of an associated graded algebra – namely, AssociatedGradedAlgebra(self), where AssociatedGradedAlgebra is AssociatedGradedAlgebra. But many instances of FilteredAlgebrasWithBasis override this method, as the associated graded algebra often is (isomorphic) to a simpler object (for instance, the associated graded algebra of a graded algebra can be identified with the graded algebra itself). Generic code that uses associated graded algebras (such as the code of the induced_graded_map() method below) should make sure to only communicate with them via the to_graded_conversion(), from_graded_conversion(), and projection() methods (in particular, do not expect there to be a conversion from self to self.graded_algebra(); this currently does not work for Clifford algebras). Similarly, when overriding graded_algebra(), make sure to accordingly redefine these three methods, unless their definitions below still apply to your case (this will happen whenever the basis of your graded_algebra() has the same indexing set as self, and the partition of this indexing set according to degree is the same as for self).

Todo

Maybe the thing about the conversion from self to self.graded_algebra() on the Clifford at least could be made to work? (I would still warn the user against ASSUMING that it must work – as there is probably no way to guarantee it in all cases, and we shouldn’t require users to mess with element constructors.)

EXAMPLES:

sage: A = AlgebrasWithBasis(ZZ).Filtered().example()
Graded Algebra of An example of a filtered algebra with basis:
the universal enveloping algebra of
Lie algebra of RR^3 with cross product over Integer Ring

induced_graded_map(other, f)

Return the graded linear map between the associated graded algebras of self and other canonically induced by the filtration-preserving map f : self -> other.

Let $$A$$ and $$B$$ be two filtered algebras with basis, and let $$(F_i)_{i \in I}$$ and $$(G_i)_{i \in I}$$ be their filtrations. Let $$f : A \to B$$ be a linear map which preserves the filtration (i.e., satisfies $$f(F_i) \subseteq G_i$$ for all $$i \in I$$). Then, there is a canonically defined graded linear map $$\operatorname{gr} f : \operatorname{gr} A \to \operatorname{gr} B$$ which satisfies

$(\operatorname{gr} f) (p_i(a)) = p_i(f(a)) \qquad \text{for all } i \in I \text{ and } a \in F_i ,$

where the $$p_i$$ on the left hand side is the canonical projection from $$F_i$$ onto the $$i$$-th graded component of $$\operatorname{gr} A$$, while the $$p_i$$ on the right hand side is the canonical projection from $$G_i$$ onto the $$i$$-th graded component of $$\operatorname{gr} B$$.

INPUT:

• other – a filtered algebra with basis
• f – a filtration-preserving linear map from self to other (can be given as a morphism or as a function)

OUTPUT:

The graded linear map $$\operatorname{gr} f$$.

EXAMPLES:

Example 1.

We start with the universal enveloping algebra of the Lie algebra $$\RR^3$$ (with the cross product serving as Lie bracket):

sage: A = AlgebrasWithBasis(QQ).Filtered().example(); A
An example of a filtered algebra with basis: the
universal enveloping algebra of Lie algebra of RR^3
with cross product over Rational Field
sage: M = A.indices(); M
Free abelian monoid indexed by {'x', 'y', 'z'}
sage: x,y,z = [A.basis()[M.gens()[i]] for i in "xyz"]


Let us define a stupid filtered map from A to itself:

sage: def map_on_basis(m):
....:     d = m.dict()
....:     i = d.get('x', 0); j = d.get('y', 0); k = d.get('z', 0)
....:     g = (y ** (i+j)) * (z ** k)
....:     if i > 0:
....:         g += i * (x ** (i-1)) * (y ** j) * (z ** k)
....:     return g
sage: f = A.module_morphism(on_basis=map_on_basis,
....:                       codomain=A)
sage: f(x)
U['y'] + 1
sage: f(x*y*z)
U['y']^2*U['z'] + U['y']*U['z']
sage: f(x*x*y*z)
U['y']^3*U['z'] + 2*U['x']*U['y']*U['z']
sage: f(A.one())
1
sage: f(y*z)
U['y']*U['z']


(There is nothing here that is peculiar to this universal enveloping algebra; we are only using its module structure, and we could just as well be using a polynomial algebra in its stead.)

We now compute $$\operatorname{gr} f$$

sage: grA = A.graded_algebra(); grA
Graded Algebra of An example of a filtered algebra with
basis: the universal enveloping algebra of Lie algebra
of RR^3 with cross product over Rational Field
sage: xx, yy, zz = [A.to_graded_conversion()(i) for i in [x, y, z]]
sage: xx+yy*zz
bar(U['y']*U['z']) + bar(U['x'])
sage: grf = A.induced_graded_map(A, f); grf
Generic endomorphism of Graded Algebra of An example
of a filtered algebra with basis: the universal
enveloping algebra of Lie algebra of RR^3 with cross
product over Rational Field
sage: grf(xx)
bar(U['y'])
sage: grf(xx*yy*zz)
bar(U['y']^2*U['z'])
sage: grf(xx*xx*yy*zz)
bar(U['y']^3*U['z'])
sage: grf(grA.one())
bar(1)
sage: grf(yy*zz)
bar(U['y']*U['z'])
sage: grf(yy*zz-2*yy)
bar(U['y']*U['z']) - 2*bar(U['y'])


Example 2.

We shall now construct $$\operatorname{gr} f$$ for a different map $$f$$ out of the same A; the new map $$f$$ will lead into a graded algebra already, namely into the algebra of symmetric functions:

sage: h = SymmetricFunctions(QQ).h()
sage: def map_on_basis(m):  # redefining map_on_basis
....:     d = m.dict()
....:     i = d.get('x', 0); j = d.get('y', 0); k = d.get('z', 0)
....:     g = (h[1] ** i) * (h[2] ** (floor(j/2))) * (h[3] ** (floor(k/3)))
....:     g += i * (h[1] ** (i+j+k))
....:     return g
sage: f = A.module_morphism(on_basis=map_on_basis,
....:                       codomain=h)  # redefining f
sage: f(x)
2*h[1]
sage: f(y)
h[]
sage: f(z)
h[]
sage: f(y**2)
h[2]
sage: f(x**2)
3*h[1, 1]
sage: f(x*y*z)
h[1] + h[1, 1, 1]
sage: f(x*x*y*y*z)
2*h[1, 1, 1, 1, 1] + h[2, 1, 1]
sage: f(A.one())
h[]


The algebra h of symmetric functions in the $$h$$-basis is already graded, so its associated graded algebra is implemented as itself:

sage: grh = h.graded_algebra(); grh is h
True
sage: grf = A.induced_graded_map(h, f); grf
Generic morphism:
From: Graded Algebra of An example of a filtered
algebra with basis: the universal enveloping
algebra of Lie algebra of RR^3 with cross
product over Rational Field
To:   Symmetric Functions over Rational Field
in the homogeneous basis
sage: grf(xx)
2*h[1]
sage: grf(yy)
0
sage: grf(zz)
0
sage: grf(yy**2)
h[2]
sage: grf(xx**2)
3*h[1, 1]
sage: grf(xx*yy*zz)
h[1, 1, 1]
sage: grf(xx*xx*yy*yy*zz)
2*h[1, 1, 1, 1, 1]
sage: grf(grA.one())
h[]


Example 3.

After having had a graded algebra as the codomain, let us try to have one as the domain instead. Our new f will go from h to A:

sage: def map_on_basis(lam):  # redefining map_on_basis
....:     return x ** (sum(lam)) + y ** (len(lam))
sage: f = h.module_morphism(on_basis=map_on_basis,
....:                       codomain=A)  # redefining f
sage: f(h[1])
U['x'] + U['y']
sage: f(h[2])
U['x']^2 + U['y']
sage: f(h[1, 1])
U['x']^2 + U['y']^2
sage: f(h[2, 2])
U['x']^4 + U['y']^2
sage: f(h[3, 2, 1])
U['x']^6 + U['y']^3
sage: f(h.one())
2
sage: grf = h.induced_graded_map(A, f); grf
Generic morphism:
From: Symmetric Functions over Rational Field
in the homogeneous basis
To:   Graded Algebra of An example of a filtered
algebra with basis: the universal enveloping
algebra of Lie algebra of RR^3 with cross
product over Rational Field
sage: grf(h[1])
bar(U['x']) + bar(U['y'])
sage: grf(h[2])
bar(U['x']^2)
sage: grf(h[1, 1])
bar(U['x']^2) + bar(U['y']^2)
sage: grf(h[2, 2])
bar(U['x']^4)
sage: grf(h[3, 2, 1])
bar(U['x']^6)
sage: grf(h.one())
2*bar(1)


Example 4.

The construct $$\operatorname{gr} f$$ also makes sense when $$f$$ is a filtration-preserving map between graded algebras.

sage: def map_on_basis(lam):  # redefining map_on_basis
....:     return h[lam] + h[len(lam)]
sage: f = h.module_morphism(on_basis=map_on_basis,
....:                       codomain=h)  # redefining f
sage: f(h[1])
2*h[1]
sage: f(h[2])
h[1] + h[2]
sage: f(h[1, 1])
h[1, 1] + h[2]
sage: f(h[2, 1])
h[2] + h[2, 1]
sage: f(h.one())
2*h[]
sage: grf = h.induced_graded_map(h, f); grf
Generic endomorphism of Symmetric Functions over Rational
Field in the homogeneous basis
sage: grf(h[1])
2*h[1]
sage: grf(h[2])
h[2]
sage: grf(h[1, 1])
h[1, 1] + h[2]
sage: grf(h[2, 1])
h[2, 1]
sage: grf(h.one())
2*h[]


Example 5.

For another example, let us compute $$\operatorname{gr} f$$ for a map $$f$$ between two Clifford algebras:

sage: Q = QuadraticForm(ZZ, 2, [1,2,3])
sage: B = CliffordAlgebra(Q, names=['u','v']); B
The Clifford algebra of the Quadratic form in 2
variables over Integer Ring with coefficients:
[ 1 2 ]
[ * 3 ]
sage: m = Matrix(ZZ, [[1, 2], [1, -1]])
sage: f = B.lift_module_morphism(m, names=['x','y'])
sage: A = f.domain(); A
The Clifford algebra of the Quadratic form in 2
variables over Integer Ring with coefficients:
[ 6 0 ]
[ * 3 ]
sage: x, y = A.gens()
sage: f(x)
u + v
sage: f(y)
2*u - v
sage: f(x**2)
6
sage: f(x*y)
-3*u*v + 3
The exterior algebra of rank 2 over Integer Ring
x
y
x*y
x*y + 1
x*y + 1
sage: A.projection(2)(x*y+1)
x*y
sage: A.projection(1)(x+2*y-2)
x + 2*y
sage: grf = A.induced_graded_map(B, f); grf
Generic morphism:
From: The exterior algebra of rank 2 over Integer Ring
To:   The exterior algebra of rank 2 over Integer Ring
u + v
2*u - v
6
-3*u*v
sage: grf(grA.one())
1

projection(i)

Return the $$i$$-th projection $$p_i : F_i \to G_i$$ (in the notations of the class documentation AssociatedGradedAlgebra, where $$A =$$).

This method actually does not return the map $$p_i$$ itself, but an extension of $$p_i$$ to the whole $$R$$-module $$A$$. This extension is the composition of the $$R$$-module isomorphism $$A \to \operatorname{gr} A$$ with the canonical projection of the graded $$R$$-module $$\operatorname{gr} A$$ onto its $$i$$-th graded component $$G_i$$. The codomain of this map is $$\operatorname{gr} A$$, although its actual image is $$G_i$$. The map $$p_i$$ is obtained from this map by restricting its domain to $$F_i$$ and its image to $$G_i$$.

EXAMPLES:

sage: A = Algebras(QQ).WithBasis().Filtered().example()
sage: p = A.an_element() + A.algebra_generators()['x'] + 2; p
U['x']^2*U['y']^2*U['z']^3 + 3*U['x'] + 3*U['y'] + 3
sage: q = A.projection(7)(p); q
bar(U['x']^2*U['y']^2*U['z']^3)
True
sage: A.projection(8)(p)
0

to_graded_conversion()

Return the canonical $$R$$-module isomorphism $$A \to \operatorname{gr} A$$ induced by the basis of $$A$$ (where $$A =$$).

This is an isomorphism of $$R$$-modules, not of algebras. See the class documentation AssociatedGradedAlgebra.

EXAMPLES:

sage: A = Algebras(QQ).WithBasis().Filtered().example()
sage: p = A.an_element() + A.algebra_generators()['x'] + 2; p
U['x']^2*U['y']^2*U['z']^3 + 3*U['x'] + 3*U['y'] + 3