Re: Assuming to be a real number is useless even in simple cases?
- To: mathgroup at smc.vnet.net
- Subject: [mg63954] Re: Assuming to be a real number is useless even in simple cases?
- From: Bill Rowe <readnewsciv at earthlink.net>
- Date: Wed, 25 Jan 2006 04:11:24 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
On 1/24/06 at 1:30 AM, roberto at pop.it (Roberto) wrote:
>In[1]:= Assuming[x\[Element]Reals, Im[x]]
>Out[1]= Im[x]
>Why it's not zero?
I would guess that Im doesn't provide a mechanism for checking assumptions, i.e., Assuming is ignored by Im.
But, since
In[3]:=
Integrate[Exp[(-a)*x], {x, 0, Infinity}]
Out[3]=
If[Re[a] > 0, 1/a,
Integrate[E^((-a)*x),
{x, 0, Infinity},
Assumptions -> Re[a] <= 0]]
clearly differs from
In[4]:=
Assuming[a > 0, Integrate[Exp[(-a)*x], {x, 0, Infinity}]]
Out[4]=
1/a
It should be clear Assuming is not useless
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