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MathGroup Archive 2000

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Re: Assumptions ignored?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25109] Re: Assumptions ignored?
  • From: Mark Fisher <mark.fisher at atl.frb.org>
  • Date: Sun, 10 Sep 2000 03:14:32 -0400 (EDT)
  • Organization: Federal Reserve Bank of Atlanta
  • References: <8p9j95$1fo@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Using GenerateConditions -> False elminates the conditions in the
output, even if it doesn't return a completely simplified result:

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

produces 

(1 - (x*z*Sqrt[1 + z^2/x^2])/(x^2 + z^2))/x

-- Mark

Mark Fisher
Economic Advisor
Research Department
Federal Reserve Bank of Atlanta
Atlanta, GA 30303
404-521-8757 (voice)
404-521-8810 (fax)
mark.fisher at atl.frb.org

personal web page: www.markfisher.net


Jos R Bergervoet wrote:
> 
> 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 plain answer (without ifs and whens) to
> use it in further computations. How can it be obtained? (If
> possible, do not give answers about menus and mouseclicks, but
> just the lines of input to be typed. I'm not very fond of GUI's ;-)
> 
> 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


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