Karim BELABAS on Tue, 29 Jun 1999 14:59:29 +0200 (MET DST) |
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Re: Recursive plot |
[Ilya:] > Try [forget about upward bias for a moment, it is not visible with > higher resolution plotting): > > ? ploth(x=-20,20,sin(x),2) > > [ nice picture, completely wrong in the center ] > > This will happen with any odd function. in a symmetric interval > The algo of subdivision works like this: it breaks the interval into > 14 subintervals, then looks on consecuitive pairs of intervals, and > checks whether 3 points corresponding to 3 ends of these intervals are > approximately on the same line. However, the middle pair of intervals > is centered at 0, and for an odd function the points will be *always* > on the same line. Thus the middle interval is never subdivided. > > Possible solution: instead of checking consecuitive pairs of intervals > (1st and 2nd, then 3rd and 4th, then 5th and 6th, etc), check all the > adjacent pairs of intervals (1st and 2nd, then 2nd and 3rd, then 3rd > and 4th, etc). Other solution: ploth(x=-20,21,sin(x),2) or ploth(x=-20,20,sin(x),2, 7) /* any (big enough!) odd number will do */ I'm documenting your example, but I don't consider it a bug. By [craftily chosing the parameters / being extremely unlucky], you'll be able to fool the recursive engine. It is much faster but less reliable, hence one should give it another try using slightly different parameters. Karim. __ Karim Belabas email: Karim.Belabas@math.u-psud.fr Dep. de Mathematiques, Bat. 425 Universite Paris-Sud Tel: (00 33) 1 69 15 57 48 F-91405 Orsay (France) Fax: (00 33) 1 69 15 60 19 -- PARI/GP Home Page: http://hasse.mathematik.tu-muenchen.de/ntsw/pari/