On Thursday, August 27, 2015 at 6:47:00 AM UTC-7, fuglede....@gmail.com wrote: > > I ran into a problem where certain kinds of Laurent polynomials, defined > through fractions, would be coercable while some other ones, defined by > more or less the same fractions, would not be. It looks like a bug to me, > but I figured I would run it by here first. Here's a concrete example of > what I mean: > > > sage: R.<x> = LaurentPolynomialRing(ZZ) > > sage: p = (1-x^2)/(1-x) > sage: p > x + 1 > sage: p.parent() > Fraction Field of Univariate Polynomial Ring in x over Integer Ring > sage: R(p) > 1 + x > sage: R(p).parent() > Univariate Laurent Polynomial Ring in x over Integer Ring > > sage: q = (1-x^-2)/(1-x^-1) # I.e., replace x by x^-1 > sage: q > (x + 1)/x > sage: q.parent() > Fraction Field of Univariate Polynomial Ring in x over Integer Ring > sage: R(q) > TypeError: denominator must be a unit > > This is due to the conversion map, which just calls R._element_constructor in this case. The code there doesn't know about fraction fields, so it tries something generic: it tries to create an element of the underlying polynomial ring and wrap that as a laurent polynomial (which doesn't work, of course) Be careful with the // operation, by the way, because it won't necessarily give you an error if you divide by a non-unit:
sage: (x+1)^3 //(3*x) 1 + x (which is of course because "//" is not proper division in ZZ, but euclidean division) -- 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.
