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How to keep things real?

  • To: mathgroup at
  • Subject: [mg25084] How to keep things real?
  • From: Jos Bergervoet <bergervo at>
  • Date: Thu, 7 Sep 2000 22:28:37 -0400 (EDT)
  • Organization: Internet Access Eindhoven, the Netherlands
  • Sender: owner-wri-mathgroup at

With the following input: 

    Integrate[ x/Sqrt[x^2+zp^2]^3, {zp,z,Infinity},
      Assumptions -> {x>0, z>0} ]

I get:

                  2    2             2        
    Out[1]= x If[x  + z  >= 0 || Im[x ] != 0,   ...., .... ]

Why does this if-statement appear in the output? Why is Mathematica
apparently ignoring my directive that x and z are positive?

The same happens with Assumptions -> {x \[Element] Reals,
z \[Element] Reals}  and with combinations of these two versions.

I would like to have the answer without "if", to use it in
subsequent computations. Is there a way to do this?

Thanks in advance,

  Dr. Jozef R. Bergervoet                      Electromagnetism and EMC
  Philips Research Laboratories,             Eindhoven, The Netherlands
  Building WS01                                     FAX: +31-40-2742224
  E-mail: bergervo at    Phone: +31-40-2742403

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