Post new topic Reply to topic  [ 2 posts ] 
Author Message
 Post subject: Roots of rational polynomials
PostPosted: Mon Apr 24, 2017 8:49 am 

Joined: Sat Feb 18, 2017 10:08 am
Posts: 4
Consider this code:

Code:
ring A=0,x,lp;
poly f=(x-3/17)*(x+2/3);


I don't see a straightforward way to find the exact (as rational numbers) roots of f. I looked a bit at the documentation and if I use the command laguerre(f,5,1), I obtain roots up to 5 digits (decimal) precision and not really the rational roots 3/17 and -2/3. Of course I can use factorize(f) and write a code that extracts the rational roots, but I am sure (I hope) there is a cleaner way to do this. Any hints would be much appreciated. Thanks!


Report this post
Top
 Profile  
Reply with quote  
 Post subject: Re: Roots of rational polynomials
PostPosted: Mon Apr 24, 2017 9:36 am 

Joined: Sat Feb 18, 2017 10:08 am
Posts: 4
So far I am using this procedure. It could exist in some library already?

Code:
//rational roots of a rational polynomial
proc Qroots(poly f)
{
  ideal l = factorize(f,1);
  list ret;
  for (int i=1;i<=size(l);i++)
  {
    if (deg(l[i])==1)
    { //roots are rational
      if (size(l[i])==1) { ret=ret+list(0);}
      else { ret = ret + list(-leadcoef(l[i][2])/leadcoef(l[i][1])); }
    }
  }
return(ret);
}


Report this post
Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 2 posts ] 

You can post new topics in this forum
You can reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

It is currently Mon Dec 18, 2017 3:25 am
cron
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group