Indeed, the factor function of Sage is terribly buggy at the moment, see https://trac.sagemath.org/ticket/27304 The issue you report seems to have been triggered by https://trac.sagemath.org/ticket/23835 (merged in Sage 8.4.beta3) as well, since we have a correct result in Sage 8.3:
┌────────────────────────────────────────────────────────────────────┐ │ SageMath version 8.3, Release Date: 2018-08-03 │ │ Type "notebook()" for the browser-based notebook interface. │ │ Type "help()" for help. │ └────────────────────────────────────────────────────────────────────┘ sage: var('t u') (t, u) sage: L = (u + t)/(u - t) sage: factor( L.substitute(t=sqrt(u))) (u + sqrt(u))/(u - sqrt(u)) but not in Sage 8.4 and later. Eric. Le lundi 6 mai 2019 16:29:37 UTC+2, Roberto Avanzi a écrit : > > Hello > > I haven't been able to find this very same bug in the trac or in the > sage-level group, even though some related ones have been reported > > L = (u + t)/(u - t) > factor( L.substitute(t=sqrt(u))) > > yields > > (u + 1)/(u - 1) > > which is wrong (the example is small enough to be verifiable by hand). > Mathematica, OTOH > > l[t_] := (u + t)/(u - t); > Factor[l[Sqrt[u]]] > > returns the correct result > > (1 + Sqrt[u])/(-1 + Sqrt[u]) > > If this is a previously unknown bug, I would like to submit it (or if > anybody wants to submit it and inform me, they're welcome!) > > best > Roberto > -- 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.