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MathGroup Archive 2004

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Re: Re: Re: Reduce/Solve


On 10 Aug 2004, at 12:02, DrBob wrote:

> *This message was transferred with a trial version of CommuniGate(tm) 
> Pro*
> It's especially sad, coming on the heels of Andrsej's valiant DEFENSE 
> of Reduce!

But I hope you also read my last paragraph:

In this particular case I am not convinced that I should radically
modify anything in my original reply, which is that Reduce returns the
original input because it is "unsure" that it can return a complete
solution using the methods available to it.
However, perhaps I should "reduce" somewhat the degree of confidence in
this assertion. There is just one thing that makes me slightly hesitant
about it. This is the fact that Reduce usually in similar cases prints
a statement to the effect that it could not solve the problem given to
it using the methods available to it.
Since this time nothing gets printed it could either mean that this
statement is printed only in certain of this type of situations or that
indeed the reason why only the original input was returned lies
somewhere else.


I see no reason to modify anything in it. Do you?

Andrzej



>
> Bobby
>
> On Mon, 9 Aug 2004 04:29:32 -0400 (EDT), Dana DeLouis 
> <delouis at bellsouth.net> wrote:
>
>> Hello.  It is because of Mathematica's huge bug in variable names.  
>> Reduce
>> often doesn't work because of this bug.  The first thing you need to 
>> try is
>> to change the variable name.  I would first start by changing the "r" 
>> to a
>> variable name that comes near the beginning of the alphabet, like "a" 
>> (at
>> least something before the "d" in the equation).
>>
>> Taking your equation, and changing r to a:
>>
>> eq = J == J0*(1 + r/d)*Exp[-(r/d)] /. r -> a
>>
>> J == ((1 + a/d)*J0)/E^(a/d)
>>
>> Now it should work:
>>
>> tt = Reduce[eq, a]
>>
>> d != 0 && J == 0 &&
>>    (J0 == 0 || a == -d) ||
>>   C[1] ? Integers &&
>>    J != 0 && J0 != 0 &&
>>    d != 0 && a ==
>>     -d - d*ProductLog[C[1],
>>        -(J/(E*J0))]
>>
>> HTH
>> Sad, isn't it!
>>
>> Dana
>>
>>
>> "Dr A.H. Harker" <a.harker at ucl.ac.uk> wrote in message
>> news:cetf3i$6i8$1 at smc.vnet.net...
>>> Dear All,
>>>          I tend to think of Reduce as a more powerful tool than 
>>> Solve,
>>> yet with
>>>
>>> eq = J == J0 (1 + r/d) Exp[-r/d]
>>> ss = Solve[eq, r]
>>> tt = Reduce[eq, r]
>>>
>>>  I get useful output from Solve, but Reduce returns the expression
>>> unevaluated. What am I missing?  Mathematica 5.0.0.0 under Windows 
>>> XP.
>>>
>>>        Tony
>>>
>>>  Dr A.H. Harker
>>>  Director of Postgraduate Studies
>>>  Deputy Head, Condensed Matter and Materials Physics Group
>>>  Department of Physics and Astronomy
>>>  University College London
>>>  Gower Street
>>>  LONDON
>>>  WC1E  6BT
>>>  (44)(0)207 679 3404
>>>  a.harker at ucl.ac.uk
>>>
>>>
>>>
>>
>>
>>
>
>
>
> -- 
> DrBob at bigfoot.com
> www.eclecticdreams.net
>


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