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Re: Question about the derivative of Abs

  • To: mathgroup at smc.vnet.net
  • Subject: [mg106174] Re: Question about the derivative of Abs
  • From: David Bailey <dave at removedbailey.co.uk>
  • Date: Mon, 4 Jan 2010 05:58:15 -0500 (EST)
  • References: <hhkj64$53d$1@smc.vnet.net>

Sam Takoy wrote:
> Hi,
> 
> I kind of asked this question before, but in a more confusing context, 
> so now I would like to ask it by itself.
> 
> In the course of my computations, I get Abs' and Abs''. It is applied to 
> a positive number, so I think those values should be 1 and 0. However, 
> Mathematica fails to simplify it.
> 
> For example
> 
> In[34]:= Abs'[2.0]
> 
> Out[34]=
> \!\(\*SuperscriptBox["Abs", "\[Prime]",
> MultilineFunction->None]\)[2.]
> 
> 
> These expressions really mess up my answers. How do I get Mathematica to 
> do these simplifications?
> 
> Thanks
> 
> Sam
> 
You could just clean up the answer by applying a suitable transformation:

expr /. {Abs'[y_]->1,Abs''[y_]->0}


David Bailey
http://www.dbaileyconsultancy.co.uk


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