Ah, yes! There are constructions of parents where the second argument is not the base ring, and there are also constructions with higher arity.
So, I guess that, to be on the safe side, a generic change_ring method would have to check `self.construction()[1] == self.base_ring()`. Martin On Saturday 13 April 2024 at 13:30:23 UTC+2 julian...@fsfe.org wrote: > Dear Martin, > > I don't know much about this area but I don't think construction() is > always implemented like that. For example FreeAlgebra(QQ, > 2).tensor(FreeAlgebra(QQ, 2)).construction() returns a functor and two > arguments. > As a default implementation in the category this might still well work for > a lot of cases. Curiously, in the tensor example, change_ring() is actually > implemented using construction() (and fails.) > > julian > > On Saturday, April 13, 2024 at 1:58:47 PM UTC+3 axio...@yahoo.de wrote: > >> Dear all, >> >> For parents in the category `Set`, sage has a method `construction`, >> which yields either None, or a functor F and a parent R, such that F(R) >> yields the original parent, see below for the docstring. >> >> Many parents implement a method `change_ring`, which returns the parent >> with the base ring replaced. For example: >> >> sage: R.<x> = QQ[] >> sage: R.change_ring(ZZ) >> Univariate Polynomial Ring in x over Integer Ring >> >> What I wonder: shouldn't `change_ring` be also a parent method in some >> category, e.g. in Category_over_base_ring, which defaults to >> >> def change_ring(self, R): >> c = self.construction() >> if c is None: >> raise NotImplementedError >> return c[0](R) >> >> Martin >> >> Signature: Sets.ParentMethods.construction(self) >> Docstring: >> Return a pair "(functor, parent)" such that "functor(parent)" >> returns "self". If "self" does not have a functorial construction, >> return "None". >> >> EXAMPLES: >> >> sage: QQ.construction() >> (FractionField, Integer Ring) >> sage: f, R = QQ['x'].construction() >> sage: f >> Poly[x] >> sage: R >> Rational Field >> sage: f(R) >> Univariate Polynomial Ring in x over Rational Field >> >> -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-devel/6cfa3c5d-90cd-4989-8aad-81d2e006d99fn%40googlegroups.com.
