With Realizations Covariant Functorial Construction¶
See also
Sets().WithRealizations
for an introduction to realizations and with realizations.sage.categories.covariant_functorial_construction
for an introduction to covariant functorial constructions.

sage.categories.with_realizations.
WithRealizations
(self)¶ Return the category of parents in
self
endowed with multiple realizations.INPUT:
self
– a category
See also
 The documentation and code
(
sage.categories.examples.with_realizations
) ofSets().WithRealizations().example()
for more on how to use and implement a parent with several realizations.  Various use cases:
SymmetricFunctions
QuasiSymmetricFunctions
NonCommutativeSymmetricFunctions
SymmetricFunctionsNonCommutingVariables
DescentAlgebra
algebras.Moebius
IwahoriHeckeAlgebra
ExtendedAffineWeylGroup
 The Implementing Algebraic Structures thematic tutorial.
sage.categories.realizations
Note
this function is actually inserted as a method in the class
Category
(seeWithRealizations()
). It is defined here for code locality reasons.EXAMPLES:
sage: Sets().WithRealizations() Category of sets with realizations
Parent with realizations
Let us now explain the concept of realizations. A parent with realizations is a facade parent (see
Sets.Facade
) admitting multiple concrete realizations where its elements are represented. Consider for example an algebra \(A\) which admits several natural bases:sage: A = Sets().WithRealizations().example(); A The subset algebra of {1, 2, 3} over Rational Field
For each such basis \(B\) one implements a parent \(P_B\) which realizes \(A\) with its elements represented by expanding them on the basis \(B\):
sage: A.F() The subset algebra of {1, 2, 3} over Rational Field in the Fundamental basis sage: A.Out() The subset algebra of {1, 2, 3} over Rational Field in the Out basis sage: A.In() The subset algebra of {1, 2, 3} over Rational Field in the In basis sage: A.an_element() F[{}] + 2*F[{1}] + 3*F[{2}] + F[{1, 2}]
If \(B\) and \(B'\) are two bases, then the change of basis from \(B\) to \(B'\) is implemented by a canonical coercion between \(P_B\) and \(P_{B'}\):
sage: F = A.F(); In = A.In(); Out = A.Out() sage: i = In.an_element(); i In[{}] + 2*In[{1}] + 3*In[{2}] + In[{1, 2}] sage: F(i) 7*F[{}] + 3*F[{1}] + 4*F[{2}] + F[{1, 2}] sage: F.coerce_map_from(Out) Generic morphism: From: The subset algebra of {1, 2, 3} over Rational Field in the Out basis To: The subset algebra of {1, 2, 3} over Rational Field in the Fundamental basis
allowing for mixed arithmetic:
sage: (1 + Out.from_set(1)) * In.from_set(2,3) Out[{}] + 2*Out[{1}] + 2*Out[{2}] + 2*Out[{3}] + 2*Out[{1, 2}] + 2*Out[{1, 3}] + 4*Out[{2, 3}] + 4*Out[{1, 2, 3}]
In our example, there are three realizations:
sage: A.realizations() [The subset algebra of {1, 2, 3} over Rational Field in the Fundamental basis, The subset algebra of {1, 2, 3} over Rational Field in the In basis, The subset algebra of {1, 2, 3} over Rational Field in the Out basis]
Instead of manually defining the shorthands
F
,In
, andOut
, as above one can just do:sage: A.inject_shorthands() Defining F as shorthand for The subset algebra of {1, 2, 3} over Rational Field in the Fundamental basis Defining In as shorthand for The subset algebra of {1, 2, 3} over Rational Field in the In basis Defining Out as shorthand for The subset algebra of {1, 2, 3} over Rational Field in the Out basis
Rationale
Besides some goodies described below, the role of \(A\) is threefold:
 To provide, as illustrated above, a single entry point for the algebra as a whole: documentation, access to its properties and different realizations, etc.
 To provide a natural location for the initialization of the bases and the coercions between, and other methods that are common to all bases.
 To let other objects refer to \(A\) while allowing elements to be represented in any of the realizations.
We now illustrate this second point by defining the polynomial ring with coefficients in \(A\):
sage: P = A['x']; P Univariate Polynomial Ring in x over The subset algebra of {1, 2, 3} over Rational Field sage: x = P.gen()
In the following examples, the coefficients turn out to be all represented in the \(F\) basis:
sage: P.one() F[{}] sage: (P.an_element() + 1)^2 F[{}]*x^2 + 2*F[{}]*x + F[{}]
However we can create a polynomial with mixed coefficients, and compute with it:
sage: p = P([1, In[{1}], Out[{2}] ]); p Out[{2}]*x^2 + In[{1}]*x + F[{}] sage: p^2 Out[{2}]*x^4 + (8*In[{}] + 4*In[{1}] + 8*In[{2}] + 4*In[{3}]  4*In[{1, 2}]  2*In[{1, 3}]  4*In[{2, 3}] + 2*In[{1, 2, 3}])*x^3 + (F[{}] + 3*F[{1}] + 2*F[{2}]  2*F[{1, 2}]  2*F[{2, 3}] + 2*F[{1, 2, 3}])*x^2 + (2*F[{}] + 2*F[{1}])*x + F[{}]
Note how each coefficient involves a single basis which need not be that of the other coefficients. Which basis is used depends on how coercion happened during mixed arithmetic and needs not be deterministic.
One can easily coerce all coefficient to a given basis with:
sage: p.map_coefficients(In) (4*In[{}] + 2*In[{1}] + 4*In[{2}] + 2*In[{3}]  2*In[{1, 2}]  In[{1, 3}]  2*In[{2, 3}] + In[{1, 2, 3}])*x^2 + In[{1}]*x + In[{}]
Alas, the natural notation for constructing such polynomials does not yet work:
sage: In[{1}] * x Traceback (most recent call last): ... TypeError: unsupported operand parent(s) for *: 'The subset algebra of {1, 2, 3} over Rational Field in the In basis' and 'Univariate Polynomial Ring in x over The subset algebra of {1, 2, 3} over Rational Field'
The category of realizations of \(A\)
The set of all realizations of \(A\), together with the coercion morphisms is a category (whose class inherits from
Category_realization_of_parent
):sage: A.Realizations() Category of realizations of The subset algebra of {1, 2, 3} over Rational Field
The various parent realizing \(A\) belong to this category:
sage: A.F() in A.Realizations() True
\(A\) itself is in the category of algebras with realizations:
sage: A in Algebras(QQ).WithRealizations() True
The (mostly technical)
WithRealizations
categories are the analogs of the*WithSeveralBases
categories in MuPADCombinat. They provide support tools for handling the different realizations and the morphisms between them.Typically,
VectorSpaces(QQ).FiniteDimensional().WithRealizations()
will eventually be in charge, whenever a coercion \(\phi: A\mapsto B\) is registered, to register \(\phi^{1}\) as coercion \(B \mapsto A\) if there is none defined yet. To achieve this,FiniteDimensionalVectorSpaces
would provide a nested classWithRealizations
implementing the appropriate logic.WithRealizations
is aregressive covariant functorial construction
. On our example, this simply means that \(A\) is automatically in the category of rings with realizations (covariance):sage: A in Rings().WithRealizations() True
and in the category of algebras (regressiveness):
sage: A in Algebras(QQ) True
Note
For
C
a category,C.WithRealizations()
in fact callssage.categories.with_realizations.WithRealizations(C)
. The later is responsible for building the hierarchy of the categories with realizations in parallel to that of their base categories, optimizing away those categories that do not provide aWithRealizations
nested class. Seesage.categories.covariant_functorial_construction
for the technical details.Note
Design question: currently
WithRealizations
is a regressive construction. That isself.WithRealizations()
is a subcategory ofself
by default:sage: Algebras(QQ).WithRealizations().super_categories() [Category of algebras over Rational Field, Category of monoids with realizations, Category of additive unital additive magmas with realizations]
Is this always desirable? For example,
AlgebrasWithBasis(QQ).WithRealizations()
should certainly be a subcategory ofAlgebras(QQ)
, but not ofAlgebrasWithBasis(QQ)
. This is becauseAlgebrasWithBasis(QQ)
is specifying something about the concrete realization.

class
sage.categories.with_realizations.
WithRealizationsCategory
(category, *args)¶ Bases:
sage.categories.covariant_functorial_construction.RegressiveCovariantConstructionCategory
An abstract base class for all categories of parents with multiple realizations.
See also
The role of this base class is to implement some technical goodies, such as the name for that category.