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  • To: mathgroup at
  • Subject: Simplifying
  • From: WTG <WTG at>
  • Date: Mon, 1 Jun 1992 15:58 MDT

Content-type: TEXT

Content-type: TEXT

Has anyone encountered, or have any ideas about resolving
the following problem? In trying to evaluate symbolically
a number of complicated expressions, I have various functions
of the following type,

In[1]:= f[x_,y_,z_] := Sqrt[m^2 + x^2 + y^2 + z^2]

that must be differentiated:

In[2]:= D[f[x,y,z], y]

Out[2]= -----------------------
              2    2    2    2
        Sqrt[m  + x  + y  + z ]

While Mathematica (v 1.2 or v 2.0) recognizes the obvious,

In[3]:= D[f[x,y,z], y] == y/f[x,y,z]

Out[3]= True

it doesn't seem to want to regroup:

In[4]:= D[f[x,y,z], y] /. Sqrt[m^2+x^2+y^2+z^2] -> f[x,y,z]

Out[4]= -----------------------
              2    2    2    2
        Sqrt[m  + x  + y  + z ]

despite considerable effort at variations on this theme.
Without being able to contract things like this, and other
types of expression, calculations become imposssibly long
and cumbersome, and the output is useless. Any hints, 
ideas, or sympathetic comments out there?

Tom Grandy
wtg at


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