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MathGroup Archive 2000

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Re: How to keep things real?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25116] Re: [mg25084] How to keep things real?
  • From: Cheng Liu <cliu at lanl.gov>
  • Date: Sun, 10 Sep 2000 03:14:38 -0400 (EDT)
  • References: <200009080228.WAA01458@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi, Jos,

	Since z appears in the integration limit, the Assumptions in
the Integrate command has no effect on it.  You can use the construct
Simplify[expr, assump] in your case:

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

Cheng


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

-- 

***********************************************

Cheng Liu
MST-8, MS-G755
Materials Science and Technology Division
Los Alamos National Laboratory
Los Alamos, NM87545

e-mail: cliu at lanl.gov
Phone:  (505)665-6892
Fax:    (505)667-8021

***********************************************


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