Re: Extracting free parameters from a Solve result

• To: mathgroup at smc.vnet.net
• Subject: [mg93932] Re: Extracting free parameters from a Solve result
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Fri, 28 Nov 2008 07:12:13 -0500 (EST)
• Organization: Uni Leipzig
• References: <ggog1b\$s65\$1@smc.vnet.net>

```Hi,

expr = {{W11 -> W44, W12 -> -W34, W13 -> W34, W14 -> -W23 - W33 + W44,
W22 -> W33, W24 -> -W34}};

Union[Cases[expr /. (_ -> a_) :> a, _Symbol, Infinity]]

??

Regards
Jens

> In solving a Sylvester matrix equation, Solve is given
> 18 equations in 10 variables: v= {W11,W12,W13,W14,W22,
> W23,W24,W33,W34,W44}  It returns
>
>  {{W11->W44, W12->-W34, W13->W34,
>   W14->-W23-W33+W44, W22->W33, W24->-W34}}
>
> How do I extract the 4 variables: W44,W34,W23,W33 left
> over on the right of the ->, and place them in an ordered list:
>
> pars={W23,W33,W34,W44}
>
> I guess the simplest way would be to Drop the dependent
> variables from the v list but the details escape me.  Thanks.
>

```

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