Examples of graded modules with basis¶

sage.categories.examples.graded_modules_with_basis.
Example
¶ alias of
sage.categories.examples.graded_modules_with_basis.GradedPartitionModule

class
sage.categories.examples.graded_modules_with_basis.
GradedPartitionModule
(base_ring)¶ Bases:
sage.combinat.free_module.CombinatorialFreeModule
This class illustrates an implementation of a graded module with basis: the free module over partitions.
INPUT:
R
– base ring
The implementation involves the following:
A choice of how to represent elements. In this case, the basis elements are partitions. The algebra is constructed as a
CombinatorialFreeModule
on the set of partitions, so it inherits all of the methods for such objects, and has operations like addition already defined.sage: A = GradedModulesWithBasis(QQ).example()
A basis function  this module is graded by the nonnegative integers, so there is a function defined in this module, creatively called
basis()
, which takes an integer \(d\) as input and returns a family of partitions representing a basis for the algebra in degree \(d\).sage: A.basis(2) Lazy family (Term map from Partitions to An example of a graded module with basis: the free module on partitions over Rational Field(i))_{i in Partitions of the integer 2} sage: A.basis(6)[Partition([3,2,1])] P[3, 2, 1]
If the algebra is called
A
, then its basis function is stored asA.basis
. Thus the function can be used to find a basis for the degree \(d\) piece: essentially, just callA.basis(d)
. More precisely, callx
for eachx
inA.basis(d)
.sage: [m for m in A.basis(4)] [P[4], P[3, 1], P[2, 2], P[2, 1, 1], P[1, 1, 1, 1]]
For dealing with basis elements:
degree_on_basis()
, and_repr_term()
. The first of these defines the degree of any monomial, and then thedegree
method for elements – see the next item – uses it to compute the degree for a linear combination of monomials. The last of these determines the print representation for monomials, which automatically produces the print representation for general elements.sage: A.degree_on_basis(Partition([4,3])) 7 sage: A._repr_term(Partition([4,3])) 'P[4, 3]'
There is a class for elements, which inherits from
IndexedFreeModuleElement
. An element is determined by a dictionary whose keys are partitions and whose corresponding values are the coefficients. The class implements two things: anis_homogeneous
method and adegree
method.sage: p = A.monomial(Partition([3,2,1])); p P[3, 2, 1] sage: p.is_homogeneous() True sage: p.degree() 6

basis
(d=None)¶ Return the basis for (the
d
th homogeneous component of)self
.INPUT:
d
– (optional, defaultNone
) nonnegative integer orNone
OUTPUT:
If
d
isNone
, returns the basis of the module. Otherwise, returns the basis of the homogeneous component of degreed
(i.e., the subfamily of the basis of the whole module which consists only of the basis vectors lying in \(F_d \setminus \bigcup_{i<d} F_i\)).The basis is always returned as a family.
EXAMPLES:
sage: A = ModulesWithBasis(ZZ).Filtered().example() sage: A.basis(4) Lazy family (Term map from Partitions to An example of a filtered module with basis: the free module on partitions over Integer Ring(i))_{i in Partitions of the integer 4}
Without arguments, the full basis is returned:
sage: A.basis() Lazy family (Term map from Partitions to An example of a filtered module with basis: the free module on partitions over Integer Ring(i))_{i in Partitions} sage: A.basis() Lazy family (Term map from Partitions to An example of a filtered module with basis: the free module on partitions over Integer Ring(i))_{i in Partitions}
Checking this method on a filtered algebra. Note that this will typically raise a
NotImplementedError
when this feature is not implemented.sage: A = AlgebrasWithBasis(ZZ).Filtered().example() sage: A.basis(4) Traceback (most recent call last): ... NotImplementedError: infinite set
Without arguments, the full basis is returned:
sage: A.basis() Lazy family (Term map from Free abelian monoid indexed by {'x', 'y', 'z'} to An example of a filtered algebra with basis: the universal enveloping algebra of Lie algebra of RR^3 with cross product over Integer Ring(i))_{i in Free abelian monoid indexed by {'x', 'y', 'z'}}
An example with a graded algebra:
sage: E.<x,y> = ExteriorAlgebra(QQ) sage: E.basis() Lazy family (Term map from Subsets of {0, 1} to The exterior algebra of rank 2 over Rational Field(i))_{i in Subsets of {0, 1}}

degree_on_basis
(t)¶ The degree of the element determined by the partition
t
in this graded module.INPUT:
t
– the index of an element of the basis of this module, i.e. a partition
OUTPUT: an integer, the degree of the corresponding basis element
EXAMPLES:
sage: A = GradedModulesWithBasis(QQ).example() sage: A.degree_on_basis(Partition((2,1))) 3 sage: A.degree_on_basis(Partition((4,2,1,1,1,1))) 10 sage: type(A.degree_on_basis(Partition((1,1)))) <type 'sage.rings.integer.Integer'>