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

**References**:**How to keep things real?***From:*Jos Bergervoet <bergervo@iaehv.iae.nl>