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Re: Full simplify problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122282] Re: Full simplify problem
  • From: dimitris <dimmechan at yahoo.com>
  • Date: Sun, 23 Oct 2011 06:24:44 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <j7u59t$u$1@smc.vnet.net>

On Oct 22, 1:18 pm, "A. Lapraitis" <ffcita... at gmail.com> wrote:
> Hello,
>
> Could anyone explain why the following does not give zero?
>
> In[72]:= Assuming[
>  x == y + z,
>  FullSimplify[
>   E^x - E^(y + z)
>   ]
>  ]
>
> Out[72]= E^x - E^(y + z)
>
> Thanks!

Actually private communication with Andrzej Kozlowski   made me
understand that the approach in my second message was incorrect.
For example each of the following evaluate to True

In[116]:= Assuming[x === y + z, FullSimplify[E^x - E^(y + z) ===
 0]]

During evaluation of In[116]:= $Assumptions::fas: Warning: One or more
assumptions evaluated to False. >>

Out[116]= True

In[119]:= Assuming[x === y + z, FullSimplify[E^x - E^(y + z) < 0]]

During evaluation of In[119]:= $Assumptions::fas: Warning: One or more
assumptions evaluated to False. >>

Out[119]= True

In[120]:= Assuming[x === y + z, FullSimplify[E^x - E^(y + z) > 0]]

During evaluation of In[120]:= $Assumptions::fas: Warning: One or more
assumptions evaluated to False. >>

Out[120]= True

Assuming False every predicate becomes True.

Of course the approach in the first message is correct.

Dimitris



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