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Re: Re: Why can't Mathematica tell when something is algebraically

• To: mathgroup at smc.vnet.net
• Subject: [mg108204] Re: [mg108148] Re: [mg108074] Why can't Mathematica tell when something is algebraically
• From: "David Park" <djmpark at comcast.net>
• Date: Wed, 10 Mar 2010 01:47:28 -0500 (EST)
• References: <201003081109.GAA03756@smc.vnet.net> <2870450.1268136686103.JavaMail.root@n11>

In view of the counterexamples that seems like a bug.


David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/  


From: leigh pascoe [mailto:leigh at evry.inserm.fr] 


Le 08/03/2010 12:09, mmdanziger a =C3=A9crit :
> This isn't the first time that I've encountered something like this in
> Mathematica but in my calculations I got a term like this:
>
> r^2 Sqrt[(r^3 + r + 2)/r] - Sqrt[r^3 (r^3 + r + 2)]
>
> Which is obviously identically zero.  For some reason Simplify or even
> FullSimplify can't figure this out.  Once you get dependent on
> Mathematica these things are pretty disturbing...you forget about your
> own knowledge because the program tells you that things are
> different.  Then you sit there like an idiot checking an algebraic
> identity that any beginning precalc student should be able to solve no
> problem.
>
> Is there any way to get Mathematica to "wake up" to these things?  It
> has such a powerful algebraic engine for most things, why can't it see
> something simple like the above?  Do you really have to manually
> override and tell the program when things should be zero?
>
> For the time being I'll just sift through and test things by hand but
> I can't believe that there isn't a better way.
>
> Best,
> md
>
>
>   
In[2]:= FullSimplify[r^2 Sqrt[(r^3+r+2)/r]-Sqrt[r^3 (r^3+r+2)]==0]
Out[2]= True

In[4]:= $Version
Out[4]= 7.0 for Microsoft Windows (32-bit) (February 18, 2009)

Leigh

• Follow-Ups:
  • Re: Why can't Mathematica tell when something is algebraically
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>

• References:
  • Why can't Mathematica tell when something is algebraically zero?
    • From: mmdanziger <mmdanziger@gmail.com>