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MathGroup Archive 1990

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Power[] corrupts, Absolute[Power[]] corrupts absolutely

  • To: mathgroup at yoda.ncsa.uiuc.edu
  • Subject: Power[] corrupts, Absolute[Power[]] corrupts absolutely
  • From: fateman at peoplesparc.Berkeley.EDU (Richard Fateman)
  • Date: Tue, 11 Dec 90 11:05:45 PST

There is some heuristic in Mma to the effect that
some functions have ``known'' derivatives, and some do not.
For example, in version 1.2, D[Abs[x],x] is Abs`[x]. 
It appears that a user definition of (appx the same)  absolute value
function by dividing the domain, say

f[x_?NonNegative]:=x
f[x_?Negative]:=-x

also cannot be differentiated  (that is, D[f[x],x] = f'[x]).
Mma presumably decided that f's derivative was unknown,
based on the information available. OK so far.

Consider yet another version of the absolute value
function function 

g[x_]:=If[x>0,x,-x]

which Mma thinks CAN be differentiated.  
It gives a mess (at least in version 1.2)
involving the derivative of ``If'' with respect to each of its
arguments. This is not too useful.  
Also, if you try evaluating g at some unexpected places, you may be surprised.
g[y] evaluates to  If[y>0,y,-y].
g[3+4I] gives an error message
 Greater::nord: Comparison with complex number 3+4 I attempted.

In an attempt to suppress this message, and also to make g[y]
come out simpler, consider this redefinition:

g[x_]:=If[x>0,x,-x,Abs[x]]

Here, the last clause to the If
is supposed to provide a result in case the test (x>0, in this case)
``gives neither True nor False''.  
Unfortunately it doesn't always work as expected.  
Now g[y] returns Abs[y]. That's ok.  But
g[3+4I] gives the same error message. [The fix, if one were to
be using Lisp, is to provide an ``errorset'' around the conditional
to convert all errors into appropriate values.]

Now consider the definitions
r[x_]:= Sqrt[x]
s[x_]:= x^2

Of course, well known to readers of this list,
we can Plot[r[s[x]],{x,-1,1}] to see that it looks exactly like
Abs[x]. Yet r[s[x]] simplifies to just x. And D[r[s[x]],x]
simplifies to just 1. This is unreasonable since r[s[-3]]
simplifies to 3, not -3.

Power corrupts...
Seasons greetings.

Richard Fateman (fateman at peoplesparc.Berkeley.edu)


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