Re: Integrating using If[ ]

• To: mathgroup at smc.vnet.net
• Subject: [mg33380] Re: Integrating using If[ ]
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Mon, 18 Mar 2002 23:38:52 -0500 (EST)
• Organization: Universitaet Leipzig
• References: <a6pol7\$bdt\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

Integrate[Exp[-Abs[t]]*Exp[-s t], {t, -Infinity, Infinity}]

work fine.

Regards
Jens

Michael Chang wrote:
>
> Hi everyone,
>
> While recently trying out my spiffy new version of Mathematica 4.1 for
> Windoze XP, I've stumbled upon the following perplexing result.
>
> Suppose I define
>
> In[1]:= x[t_]:=If[t>=0,Exp[-t],Exp[t]];
>
> If I now try and evaluate (a two-sided Laplace Transform)
>
> In[2]:= Integrate[x[t] Exp[-s t],{t,-Infinity,Infinity}]
>
> all I get back is essentially an unevaluated answer.  Replacing
> {-Infinity, Infinity} by, for instance, {-100,100}, *does* give me an
>
> Typing in
>
> In[3]:= Integrate[Exp[-Abs[t]] Exp[-s t],{t,-Infinity,Infinity}]
>
> however, does give me back a meaningful result with the conditionals
> properly stated that -1<Re[s]<1.  Why does In[3] work, but not In[1]
> and In[2]?  Am I foolishly doing something stup!d here?
>
> Michael

```

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