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MathGroup Archive 2004

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Re: Integrate UnitStep, Bug?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50328] Re: [mg50303] Integrate UnitStep, Bug?
  • From: DrBob <drbob at bigfoot.com>
  • Date: Fri, 27 Aug 2004 02:57:50 -0400 (EDT)
  • References: <200408261050.GAA16330@smc.vnet.net>
  • Reply-to: drbob at bigfoot.com
  • Sender: owner-wri-mathgroup at wolfram.com

I think both results are wrong, and the correct answer is Abs[b-a]. Unless these Plots are wrong?

fr[e_] := UnitStep[-e];
fl[e_] := UnitStep[-(e - b)];

b = 2; a = 3;
Plot[fl[e](fr[e - a] - fr[e]), {e, -5, 5}, PlotStyle -> {Thickness[0.01]}]

b = 5; a = 3;
Plot[fl[e](fr[e - a] - fr[e]), {e, -5, 5}, PlotStyle -> {Thickness[0.01]}]

Bobby

On Thu, 26 Aug 2004 06:50:47 -0400 (EDT), <jens at fika.de> wrote:

> The following code returns the incorrect result (-a+b) UnitStep[a-b]. It
> should be Min[a,b].
>
> fr[e_]=UnitStep[-e];
> fl[e_]=UnitStep[-(e-b)];
> Integrate[fl[e](fr[e-a]-fr[e]),{e,-Infinity,Infinity},Assumptions->{b>0,a>0}]
>
> Is this a known bug? Should I avoid using step functions with Integrate?
> Any comments appreciated.
>
>
>



-- 
DrBob at bigfoot.com
www.eclecticdreams.net


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