# With Realizations Covariant Functorial Construction¶

sage.categories.with_realizations.WithRealizations(self)

Return the category of parents in self endowed with multiple realizations.

INPUT:

• self – a category

Note

this function is actually inserted as a method in the class Category (see WithRealizations()). 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, and Out, 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 MuPAD-Combinat. 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 class WithRealizations implementing the appropriate logic.

WithRealizations is a regressive 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 calls sage.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 a WithRealizations nested class. See sage.categories.covariant_functorial_construction for the technical details.

Note

Design question: currently WithRealizations is a regressive construction. That is self.WithRealizations() is a subcategory of self by default:

sage: Algebras(QQ).WithRealizations().super_categories()
[Category of algebras over Rational Field,
Category of monoids with realizations,

Is this always desirable? For example, AlgebrasWithBasis(QQ).WithRealizations() should certainly be a subcategory of Algebras(QQ), but not of AlgebrasWithBasis(QQ). This is because AlgebrasWithBasis(QQ) is specifying something about the concrete realization.