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Re: Abs and derivative problems

• To: mathgroup at smc.vnet.net
• Subject: [mg14717] Re: Abs and derivative problems
• From: "A.ODA" <a-oda at muj.biglobe.ne.jp>
• Date: Tue, 10 Nov 1998 01:21:07 -0500
• Organization: BIGLOBE dial-up user
• References: <71q9qu$j25@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

sylvan wrote in message <71q9qu$j25 at smc.vnet.net>...
>Also,  replacement rules like  //. z[t_] -> t^2 do not work well on
>expressions like  z'[t] + b z[t]. the result  is  z'[t] + b t^2... I
>could not force it to Evaluate z'[t] or D[z[t], t]. 
>
In this case, you can use "Hold" and "RelesseHold".

Define expression using "Hold" to postpone evalueation like derivatives.
After assign some rules, place "ReleaseHold" at the expression.

example

expr=D[Hold[z[t]],t]+b z[t]
(expr /. z[t]->t^2)//ReleaseHold

result in 

2t + b t^2


A. ODA <a-oda at muj.biglobe.ne.jp>
Kawasaki, JAPAN