On 19 December 2012 23:16, Charles Bouillaguet <charles.bouillag...@gmail.com> wrote: > On Dec 19, 2012, at 12:44 PM, John Cremona wrote: > > > --- > Charles Bouillaguet > http://www.lifl.fr/~bouillaguet/ > > > >> I suggest adding to the documentation of A.coordinates() that it works >> also if the given element is in the ambient space, with an example of >> that. In concrete terms you need the element to be an F-linear >> combination of the generators where F is the fraction field of the >> base ring R, and not just an R-linear combination. >> >> This functionality is useful (I have used it in some number field >> functions) and I would not want it removed. > > Well, you could always access it through: > > R.ambient_vector_space().coordinates( x )
That is a good point. THere may be a few places elsewhere in the Sage library where this change would have to be made (not too many, I hope), and in those places one could argue that the code would be clearer anyway, after this change. > > One problem I can see is "what to do when the ambient vector space is not > defined?" (this happens when the ring is not a PID, because then its fraction > field is not defined). Examples include: > > sage: R.<x,y> = QQ[] > sage: I = (x*y) * R > sage: Q = R.quotient( I ) > sage: M = Q^3 > sage: M.ambient_vector_space() # BOOM > OK, so in such a case there would still be no problem asking for the coordinates of an element of M itself. Which in such a simple example is almost a tautology: the most common use case (that I can think of) is when you have a free submodule N of such an M, and an element of M which happens to lie in N, and you want its coordinates w.r.t. N. (I use this all the time for vector spaces, but the principle is the same). > This is now : http://trac.sagemath.org/sage_trac/ticket/13853 Noted. One further point: why is ArithmeticError the right exception to raise? I would have expected ValueError. John > --- > Charles Bouillaguet > http://www.lifl.fr/~bouillaguet/ > > >> John >> >> >>> --- >>> Charles Bouillaguet >>> http://www.lifl.fr/~bouillaguet/ >>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "sage-devel" group. >>> To post to this group, send email to sage-devel@googlegroups.com. >>> To unsubscribe from this group, send email to >>> sage-devel+unsubscr...@googlegroups.com. >>> Visit this group at http://groups.google.com/group/sage-devel?hl=en. >>> >>> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "sage-devel" group. >> To post to this group, send email to sage-devel@googlegroups.com. >> To unsubscribe from this group, send email to >> sage-devel+unsubscr...@googlegroups.com. >> Visit this group at http://groups.google.com/group/sage-devel?hl=en. >> >> > > -- > You received this message because you are subscribed to the Google Groups > "sage-devel" group. > To post to this group, send email to sage-devel@googlegroups.com. > To unsubscribe from this group, send email to > sage-devel+unsubscr...@googlegroups.com. > Visit this group at http://groups.google.com/group/sage-devel?hl=en. > > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To post to this group, send email to sage-devel@googlegroups.com. To unsubscribe from this group, send email to sage-devel+unsubscr...@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel?hl=en.
