Re: Abs and derivative problems
- To: mathgroup at smc.vnet.net
- 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$j25@smc.vnet.net> <728lbo$eit@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
The problem is that the FullForm of z'[t] does not contain z[t]. In[1]:= FullForm[z'[t] + b z[t]] Out[1]//FullForm= Plus[Times[b,z[t]],Derivative[1][z][t]] It does contain z, though. So you can replace z with the pure function for t^2. In[2]:= z'[t] + b z[t]/.z->(#^2&) Out[16]= 2 t + b t^2 -Dale A.ODA wrote in message <728lbo$eit at smc.vnet.net>... sylvan wrote in message <71q9qu$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