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Re: Abs and derivative problems

  • To: mathgroup at
  • Subject: [mg14946] Re: Abs and derivative problems
  • From: "Dale Horton" <daleh>
  • Date: Fri, 27 Nov 1998 03:49:43 -0500
  • Organization: Wolfram Research, Inc.
  • References: <71q9qu$> <728lbo$>
  • Sender: owner-wri-mathgroup at

The problem is that the FullForm of z'[t] does not contain z[t].

FullForm[z'[t] + b z[t]]


It does contain z, though. So you can replace z with the pure function
for t^2.

z'[t] + b z[t]/.z->(#^2&)
2 t + b t^2


A.ODA wrote in message <728lbo$eit at>...

sylvan wrote in message <71q9qu$j25 at>...
>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.


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>
Kawasaki, JAPAN

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