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