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Re: Is this normal for Limit?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg82287] Re: Is this normal for Limit?
  • From: "Kevin J. McCann" <Kevin.McCann at umbc.edu>
  • Date: Wed, 17 Oct 2007 03:54:30 -0400 (EDT)
  • Organization: University System of Maryland
  • References: <ff1par$8nq$1@smc.vnet.net>

There are two problems:

1) I think the $Pre definition below is applied to $Assumptions so when 
you look at the variable you get {t\[Element]Reals,True,True}.

2) The second problem appears to be a bug. If I reset $Pre and leave the 
$Assumptions alone, I get

Limit[\[ExponentialE]^(-k t), t -> \[Infinity]]

just repeated, i.e. Mathematica doesn't evaluate; however, if I explicitly 
include Assumptions in the call, I get the correct answer:

Limit[\[ExponentialE]^(-k t), t -> \[Infinity],
  Assumptions :> {t \[Element] Reals, k \[Element] Reals, k > 0}]

0



Michael Mandelberg wrote:
> I have what I would think is the following simple limit to evaluate:
> 
> $Pre = Refine[#] &;
> $Assumptions = {t \[Element] Reals, k \[Element] Reals,  k > 0};
> 
> In[3]:= Limit[Exp[-k t], t -> Infinity]
> 
> The results is:
> 
> Out[3]= Limit[\[ExponentialE]^(-k t), t -> \[Infinity]]
> 
> In other words Mathematica 6.0.0 punts on this.  Is there any sense in
> which this limit is not well-defined?
> 
> Michael Mandelberg
> 
> 


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