[Maxima-bugs] [ maxima-Bugs-932302 ] partfrac/ratnumer of taylor bad: taychk2rat/FIX

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Bugs item #932302, was opened at 2004-04-09 09:45
Message generated for change (Comment added) made by macrakis
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Stavros Macrakis (macrakis)
Assigned to: Nobody/Anonymous (nobody)
>Summary: partfrac/ratnumer of taylor bad: taychk2rat/FIX

Initial Comment:
  expr: 1/(x^2-1)$
  texpr: taylor(expr,x,1,1)$

  ptexpr: partfrac(texpr,x) =>  (x+4/(x-1)-3)/8  NO!

This is algebraically correct, but not in partfrac form.

The correct answer is given by:

  partfrac(ratdisrep(texpr),x)
  == partfrac(ptexpr,x)

       => 1/(2*(x-1))+(x-3)/8

The immediate fix is to replace

 (DESETQ (RATFORM . EXP) (TAYCHK2RAT EXP))

with

 (DESETQ (RATFORM . EXP) (RATF (TAYCHK2RAT EXP)))

however, I wonder if TAYCHK2RAT shouldn't be doing 
this.  Compare:

ratnumer(taylor(x+1/x,x,0,1)) => x+1/x ???


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>Comment By: Stavros Macrakis (macrakis)
Date: 2004-04-09 10:05

Message:
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user_id=588346

taychk2rat is used in four places in Maxima:

   ratnumer
   ratdenom
   partfrac
   horner

In all of them, it would be better if taychk2rat performed the 
ratf.  So the fix should be in taychk2rat, not in the callers.

Or maybe in $taytorat.  I don't understand what srrat does -- 
is that the problem?  Maybe look at this later.

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