[Maxima-bugs] [ maxima-Bugs-2024777 ] find_root() gives completely wrong result

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Bugs item #2024777, was opened at 2008-07-22 11:19
Message generated for change (Comment added) made by dgildea
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Category: Lisp Core - Solving equations
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: find_root() gives completely wrong result

Initial Comment:
I am using Maxima 5.15.0 under Windows.
 
find_root((655/656)^n = 0.5, n, 1, 1000) gives 114.8, which is completely wrong.
 
Fortunately, I checked the answer (always a good habit, I know!).
 
Either of the following expressions give the correct answer, which is 454.4
 
find_root(float(655/656)^n = 0.5, n, 1, 1000);
float(log(0.5)/log(655/656));

Maxima version: 5.15.0
Maxima build date: 17:36 4/20/2008
host type: i686-pc-mingw32
lisp-implementation-type: GNU Common Lisp (GCL)
lisp-implementation-version: GCL 2.6.8

email: tow_force at hotmail.com

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>Comment By: Dan Gildea (dgildea)
Date: 2008-10-18 17:10

Message:
Seems to be specific to GCL.
Problem is caused by floating point overflow computing 655^1000.0.

(%i7) find_root((655/656)^n = 0.5, n, 1, 1000) ;
(%o7) find_root(655^n/656^n = 0.5,n,1.0,1000.0)

(%i8) build_info();
Maxima version: 5.15.0
Maxima build date: 10:24 6/17/2008
host type: i386-redhat-linux-gnu
lisp-implementation-type: SBCL
lisp-implementation-version: 1.0.17



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You can respond by visiting: 
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