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MathGroup Archive 2004

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Re: Question: ABS and \[Prime]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47528] Re: Question: ABS and \[Prime]
  • From: wellsed at wam.umd.edu (Ed Wells)
  • Date: Thu, 15 Apr 2004 03:39:53 -0400 (EDT)
  • References: <c5gfrm$apq$1@smc.vnet.net> <c5j79i$qt8$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

"David W. Cantrell" <DWCantrell at sigmaxi.org> wrote in message news:<c5j79i$qt8$1 at smc.vnet.net>...
> wellsed at wam.umd.edu (Ed Wells) wrote:
> > In my notebook output, a copy of which can be seen at :
> >
> > www.glue.umd.edu/~wellsed/derivatives.html
> >
> > I'm finding the derivatives of a complex function involving distances
> > (R[] is a distance function) in an energy function (W[] is an energy
> > function).
> 
> Having looked at your notebook, I must guess that, when you said "complex"
> above, you simply meant "complicated". 

Yes.  My mistake :)

>Perhaps Abs was used merely to avoid
> getting a negative number under Sqrt. If that is the case, then look at the
> thread "Abs function", started earlier this month by nikmatz, at
> <http://groups.google.com/groups?threadm=c4j9sq%24dha%241%40smc.vnet.net>.

Yes, that is the intent, though I'm starting to wonder if it is truly
necessary for the purposes of establishing derivatives of this
particular function.  I've read the thread you suggested, but I'm not
quite so well-versed in Mathematica, so it will take me time to
understand the methods and issues presented.

> 
> > Each of my derivatives looks okay except for the presence
> > of the Abs[] function with a \[Prime] following it.  It does not
> > appear in every instance, but I cannot find any documentation for what
> > this may mean.  An example is in the first line of Out[36].
> 
> As explained in the cited thread, the prime mark means "derivative". That
> thread also gives ways to avoid getting Abs', assuming that your quantities
> are real numbers.

Yes, they are real numbers, specifically, they are the necessary
distance and angular measurements of a box such that the distance
between two points within that box can be found.

> 
> HTH,
> David


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