       Re: Re: Can anyone help about this inverse laplacetransform problem?

• To: mathgroup at smc.vnet.net
• Subject: [mg33316] Re: [mg33279] Re: [mg33260] Can anyone help about this inverse laplacetransform problem?
• From: Sseziwa Mukasa <mukasa at jeol.com>
• Date: Thu, 14 Mar 2002 19:51:17 -0500 (EST)
• Organization: JEOL (USA) Ltd.
• Sender: owner-wri-mathgroup at wolfram.com

```david Xia wrote:

>   Thanks for your suggestion, since the expression of fi is a rational
> expression, how to use mathematica to seperate fi into different parts
> according to its poles? Is there a function to do this automatically?
>

{ToRules[Roots[Denominator[Together[f4s]]==0,s]]}should get you the roots as a
list of replacement rules for s, the expression will be complicated.

>
>   To find the poles, for my question, the parameters are all unknown
> variables, will the find root routine find roots based on the symbolic
> function? As I understand, the root function works only for numerical
> solution by newton-raphson method given a initial value, am I right? If so,
> then how to find root then for a symbolic function?
>

Roots is explicitly for finding all the roots of a polynomial expression.
Since the denominator of the rational function is of fourth degree there is a
closed form expression for the roots, but it is complicated.

Regards,

Ssezi

```

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