Dear Jan,
On Tue, 08 Apr 2014 20:19:47 +0200, Jan <jrheinlaender@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@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