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Re: Question about the derivative of Abs

• To: mathgroup at smc.vnet.net
• Subject: [mg106116] Re: Question about the derivative of Abs
• From: dh <dh at metrohm.com>
• Date: Sat, 2 Jan 2010 05:03:19 -0500 (EST)
• References: <hhkj64$53d$1@smc.vnet.net>

Hi Sam,
mathematica does not know the derivative of Abs, but it is easy to add
a corresponding rule:

 Abs'[x_?Positive] = 1
 Abs''[x_?Positive] = 0

Daniel

On 1 Jan., 11:35, Sam Takoy <samta... at yahoo.com> 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