On Tuesday, June 12, 2018 at 2:09:53 PM UTC-7, Simon King wrote: > > > Problem: > It is currently possible to create the polynomial ring R[x][x] > (hence, x occurs twice). If "x" has two different meanings in the > same ring, the notion "name preserving map" makes no sense. I believe > it should be made impossible to create such ring. Perhaps the .__mul__ > method of PolynomialRing and MultiPolynomialRing could be utilised for > this, when needed. > > There is a very good reason to allow such rings to exist:
sage: matrix(QQ['y'],3,3,1).characteristic_polynomial() x^3 - 3*x^2 + 3*x - 1 sage: matrix(QQ['x'],3,3,1).characteristic_polynomial() x^3 - 3*x^2 + 3*x - 1 If you forbid them, then characteristic_polynomial needs to start working very hard to come up with a sensible ring. Of course, any name-based coercion discovery is allowed to fail horribly with the results, but I think we need to keep them around as place holder for these automatically constructed parents. -- 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 post to this group, send email to sage-devel@googlegroups.com. Visit this group at https://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
