Re: Question about the derivative of Abs

• To: mathgroup at smc.vnet.net
• Subject: [mg106140] Re: [mg106097] Question about the derivative of Abs
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Sat, 2 Jan 2010 05:07:54 -0500 (EST)

```sub = {
Abs'[x_?NumericQ /; x != 0] -> Sign[x],
Abs''[x_?NumericQ /; x != 0] -> 0};

Abs'[2.0] + Abs''[2.0] /. sub

1

Bob Hanlon

---- Sam Takoy <samtakoy 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

```

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