[GiNaC-devel] Derivative of abs()

[Vladimir V. Kisil]

	Dear Jan,

>>>>> On Tue, 08 Apr 2014 20:19:47 +0200, Jan <jrheinlaender at gmx.de> said:

    Jan> Hi, is there any reason not to define the derivative of the
    Jan> built-in abs() function as either abs(x)/x or csgn(x) ?
    Jan> Currently it has not derivative defined at all.

    You formula does not work for abs(f) and complex values. I have
  proposed here 

http://www.ginac.de/pipermail/ginac-devel/2013-November/002053.html

  another formula:

(abs(f))'=(f'*f.conjugate()+f*f'.conjugate())/2/abs(f)

  It requires some revision of function class and new method of
  derivation going beyond the chain rule. I proposed to do this job, but
  do not remember my email to be answered.

  Best wishes,
  Vladimir
-- 
Vladimir V. Kisil     email: kisilv at maths.leeds.ac.uk
                        www: http://www.maths.leeds.ac.uk/~kisilv/
Book: Geometry of Mobius Transformations
http://www.worldscientific.com/worldscibooks/10.1142/p835