[Maxima-bugs] [ maxima-Bugs-624061 ] strange bug (probably) in numer

[<no...@so...>]

Bugs item #624061, was opened at 2002-10-16 08:18
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Category: Lisp Core
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: strange bug (probably) in numer

Initial Comment:
Maxima version: 5.9.0rc1
Maxima build date: 11:40 9/3/2002
host type: i686-pc-linux-gnu
lisp-implementation-type: Kyoto Common Lisp
lisp-implementation-version: GCL-2-5.0

(C2) display2D:false$

(C3) f(x):=x^3-10*x^2+28.5*x-21$

(C4) rhs(solve(f(x),x)[1]),rectform,numer;


RAT replaced 28.5 by 57//2 = 28.5

RAT replaced 28.5 by 57//2 = 28.5

RAT replaced 0.5 by 1//2 = 0.5

RAT replaced -4.18055555555556 by -301//72 =
-4.18055555555556
(D4) 3.897114317029973*%I+0.75

******** which is wrong *******

(C5) y:rhs(solve(f(x),x)[1]),rectform$


RAT replaced 28.5 by 57//2 = 28.5
(C6) y,numer;

(D6) 3.318006917974609

******* which is correct ******

M.At.Stanev reports with Maxima 5.5

(C1) f(x):=x^3-10*x^2+28.5*x-21;
(D1) ...
(C2) rhs(solve(f(x),x)[1]),rectform,numer;
(D2) 12.01301985645547*%I+10.38424295325536

Martin

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Comment By: Stavros Macrakis (macrakis)
Date: 2002-11-18 11:18

Message:
Logged In: YES 
user_id=588346

First thing to note: in "foo,numer,rectform", the "numer" and 
the "rectform" actually have very different meanings.  Numer 
sets the numer flag *during* the evaluation of foo, whereas 
rectform simply calls rectform on the result of the evaluation 
of foo.  That is, "foo,numer,rectform" is equivalent to rectform
(ev(foo,numer)).

Thus, solve(...),numer,rectform calculates the roots with 
numerical, not symbolic, intermediate results, then takes the 
rectform of that.  On the other hand, y:solve(...),rectform$ 
y,numer; calculates the solutions numerically, then takes the 
symbolic rectform, then evaluates numerically.

So it is not surprising that the two results will be slightly 
different.

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Comment By: Martin Rubey (kratt5)
Date: 2002-11-18 10:34

Message:
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user_id=651552

 I replaced f* by * in mul2* in 5.9.0rc3, but got still different behaviour using the two    
approaches - the difference is rather small now, so it might be ok, but I do not    
understand why there is a difference...    
    
(C1) f(x):=x^3-10*x^2+28.5*x-21$   
   
(C2) rhs(solve(f(x),x)[1]),rectform,numer;   
   
   
RAT replaced 28.5 by 57//2 = 28.5   
   
RAT replaced 28.5 by 57//2 = 28.5   
   
RAT replaced 0.5 by 1//2 = 0.5   
   
RAT replaced -4.18055555555556 by -301//72 = -4.18055555555556   
(D2)             2.220446049250313E-16 %I + 3.318006917974608   
  
--------------------------------- whereas -----------------------------------  
  
(C3) y:rhs(solve(f(x),x)[1]),rectform$   
   
   
RAT replaced 28.5 by 57//2 = 28.5   
(C4) y,numer;   
   
(D4)                           3.318006917974609   
   

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Comment By: Stavros Macrakis (macrakis)
Date: 2002-10-21 19:32

Message:
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user_id=588346

The problem is in the function MUL2*, which uses f* to 
multiply non-fixnums: (MUL2* -1 -150.5) returns -269521184.

The correction is to replace "f*" by "*" in MUL2* (in opers.lisp).

----------------------------------------------------------------------

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