Re: Problems with elldivpol - will they be fixed in 2.6.0?

[Karim Belabas <Karim.Belabas@math.u-bordeaux1.fr>]

* Richard in Reading [2013-05-03 18:58]:
> I obtained pari-2.6.0.svn13433.tar,gz from the website and built it in an attempt to escape apparently incorrect elldivpol results which plagued previous pari versions for unix.
> By contrast I have a version [2.5.1] running on Windows which apparently doesn't suffer from the same problem in this instance.
> 
> So on the [2,6,0,"svn-13433"] version, if I do 
> 
> x=0;factor(eval(elldivpol(ellinit([0,-1,1,-10,-20]),13)))
> x=0;factor(eval(elldivpol(ellinit([0,-1,1,-10,-20]),13)))
> 
> it prints the factors 2731,185693,57084047 etc...
> however if I then do 
> 
> e=ellinit(Mod([0,-1,1,-10,-20],2731));lift(ellpow(e,[0,ellordinate(e,0)[1]],13))
> 
> it prints [1360,2238] which I believe to indicate that 2731 is incorrect. Similar checks of the other factors fail. 
> 
> On the [2,5,1] version on Windows, two of the factors are 
> 27793 and 5141213801370234316849605048349

A few minor remarks:

  * I'd use polcoeff(T, 0) or subst(T,x,0), rather than
      x = 0; eval(T)
    to get the constant coefficient.

  * In 2.6, e = ellinit([0,-1,1,-10,-20], 2731) is the recommended way
to define a curve over F_2731

Now on to your actual problem: I'm not too sure what [2,6,0,"svn-13433"] refers
to yet (we dropped svn for git a long time ago, I don't know how to translate
"SVN repository version 13433" to a git commit name at this point), but in any
case it's a very old outdated version.

In the up to date development branch for 2.6.0 (aka 'master'), version()
outputs [2, 6, 0, "git-ca538f5"], and I correctly get

  (19:39) gp > x=0;factor(eval(elldivpol(ellinit([0,-1,1,-10,-20]),13)))
  %2 = 
  [                          27793 1]

  [                      132199183 1]

  [                     3597411289 1]

  [5141213801370234316849605048349 1]

> Does anyone have a good idea of the state of bugs in elldivpol for 2.6.0 ?

No known bugs in the 'master' branch. You can check all currently open
tickets in 2.6.0 from our Bug Tracking System at

  http://pari.math.u-bordeaux.fr/cgi-bin/pkgreport.cgi?pkg=pari

(none of them concern elliptic curves)

> I would be very grateful to receive instructions on how to build a version
> that gives the correct results! 

You may either

- install pari-2.5.3 from our Download page
    http://pari.math.u-bordeaux.fr/download.html

- or give pari-2.6.0 a try (esp. for elliptic curve related work); the
  recommended way for this is to use 'git'
    http://pari.math.u-bordeaux1.fr/anongit.html
  (because bleeding edge versions may be very buggy and git simplifies updates)
  If for some reason, git doesn't work for you, a nighly snapshot is at
    http://pari.math.u-bordeaux1.fr/pub/pari/snapshots/

Feel free to report on further problems getting either versions to work.

(I'm assuming a Linux machine above; for Windows, the Download page
contains a recent snapshot that "should just work")

Cheers,

    K.B.
--
Karim Belabas, IMB (UMR 5251)  Tel: (+33) (0)5 40 00 26 17
Universite Bordeaux 1          Fax: (+33) (0)5 40 00 69 50
351, cours de la Liberation    http://www.math.u-bordeaux1.fr/~kbelabas/
F-33405 Talence (France)       http://pari.math.u-bordeaux1.fr/  [PARI/GP]
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