On Thursday, October 1, 2015 at 4:19:40 PM UTC-5, Nils Bruin wrote: > > On Thursday, October 1, 2015 at 11:57:45 AM UTC-7, Travis Scrimshaw wrote: >> >> Hey, >> You need to make sure the order of the variables agrees: >> >> sage: A = QQ['t'] >> sage: B = A['x','y'] >> sage: C = QQ['t', 'x','y'] >> sage: C.coerce(B.0) >> x >> > > That's a bug in sage. It's completely canonical in sage where names should > be coerced, and it goes by name. Not by index number. The variable order in > C should not matter in whether sage can figure out the coercion. > > > >> The reason the order is important is say you have QQ['x,y'] and >> QQ['y,x'], should the coercion be index to index or name to name? >> > > Sage firmly defines this: the coercion is by name. > > sage: U=QQ['x,y'] > sage: V=QQ['y,x'] > sage: U.0+V.1 > 2*x > sage: parent(U.0+V.1) > Multivariate Polynomial Ring in x, y over Rational Field > sage: parent(V.1+U.0) > Multivariate Polynomial Ring in y, x over Rational Field > > (as you can see, in this case there are coercions in both directions which > is a little unfortunate) > > (Although I'd advocate for multivariate polynomial rings to sort their >> generators upon creation so that QQ['x,y'] is QQ['y,x'] in the sense of >> python.) >> > > That's a bad idea: names have mathematical significance in sage as far as > coercion is concerned. If you can't rely on the order of the names, then > you don't know the mathematical meaning of R.0 as far as coercions to other > rings are concerned. Another reason to preserve given order in variable > names is that the order of the variables matters for monomial orderings: If > you want to get elimination orders in place, it's nice to know that the > order of the names of your variables is determined by what you type in. >
Ah, that right... I knew there was something with the indices being fixed. Sorry for the noise. Best, Travis -- 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 http://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
