On Mon, Jan 26, 2009 at 11:48 AM, Robert Bradshaw <rober...@math.washington.edu> wrote: > > On Jan 26, 2009, at 3:44 AM, Harald Schilly wrote: > >> >> Hi, on the public bug tracker I got this one: >> http://spreadsheets.google.com/ver? >> key=pCwvGVwSMxTzT6E2xNdo5fA&t=1232807032283000&pt=1232807012283000&dif >> fWidget=true&s=AJVazbXBr2D7KZ6E3qJBWICjRrHj5pKG-Q&pli=1 > > I didn't know there was another "public bug tracker," but that link > isn't working for me...
I think this is the "report a bug" link in the notebook. > > http://trac.sagemath.org/sage_trac/ticket/5107 > >> >> quote: >> """ >> A simple call to the function with an irrational number returns a list >> for the infinite continued fractions, with the last digit or two >> incorrect. >> >> For exapmle: >> >> continued_fraction(sqrt(2)) >> [1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1] >> >> the last two digits are incorrect >> >> continued_fraction(sqrt(109)) >> [10, 2, 3, 1, 2, 4, 1, 6, 6, 1, 4, 2, 1, 3, 2, 20, 3] >> >> the last digit (3) is incorrect >> """ >> >> Interestingly, the doctests in >> http://hg.sagemath.org/sage-main/file/b0aa7ef45b3c/sage/rings/arith.py >> / continued_fraction_list(...) >> clearly state that this is correct. >> >> If i read the code correctly, the symbolic expression is evaluated >> with limited precision. I think it should stated in the doctests that >> it is an approximation (line #2676 -> "# if x is a >> SymbolicExpression, try coercing it to a real number") - or I'm wrong >> or is this a bug? > > Yes, this is a bug. It looks like its not correctly taking into > account the uncertainty in the last digit, but it could be > intermediate rounding issues as well. The OP said: " continued_fraction(sqrt(2)) [1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1] the last two digits are incorrect" A basic property of continued fractions is that [1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1] = [1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2] so I dispute the claim that the last two digits are incorrect. > > Interestingly enough > > sage: continued_fraction(QQ(RR(sqrt(2)))) > [1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2] > > sage: continued_fraction(RR(sqrt(2)).exact_rational()) > [1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, > 1, 2, 7, 1, 2, 33, 2, 7, 5, 2, 1, 1, 16, 2] > > Of course ideally it could directly compute the continued fraction of > sqrt(2), but that wouldn't fix this bug. > > - Robert > > > > > -- William Stein Associate Professor of Mathematics University of Washington http://wstein.org --~--~---------~--~----~------------~-------~--~----~ 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 For more options, visit this group at http://groups.google.com/group/sage-devel URLs: http://www.sagemath.org -~----------~----~----~----~------~----~------~--~---
