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Re: ReplaceAll / sequential replacements versus joint replacement

• To: mathgroup at smc.vnet.net
• Subject: [mg55448] Re: [mg55429] ReplaceAll / sequential replacements versus joint replacement
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Thu, 24 Mar 2005 03:41:48 -0500 (EST)
• References: <200503231034.FAA03987@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

On 23 Mar 2005, at 11:34, Ronald Wendner wrote:

> Suppose there is an expression e1 which depends, among others, on 
> variables
> a and b.
> Replace both a and b by zero:
>
> e1/.{a->0,b->0} Mathematica gives, say, result1.
>
> Next, consider
>
> e2 = e1/.{a->0};
> e2/.{b->0} Mathematica gives, say, result2.
>
> How can result 1 differ from result 2?
>
> For my specific problem, I also attach a short Mathematica notebook.
> [Contact the author to obtain the notebook - moderator]
>
> I'd appreciate any help.
>
> Many thanks,
> Ron
>
>
They certianly can differ, for example:


(a - 2*b)/(a - b) /. {b -> 0, a -> 0}

Indeterminate

vs

(a - 2*b)/(a - b) /. a -> 0 /. b -> 0

2


Andrzej Kozlowski
Chiba, Japan
http://www.akikoz.net/andrzej/index.html
http://www.mimuw.edu.pl/~akoz/

• References:
  • ReplaceAll / sequential replacements versus joint replacement
    • From: "Ronald Wendner" <Ronald.Wendner@uni-graz.at>