Getting the Derivative of an HoldForm Expression

*To*: mathgroup at smc.vnet.net*Subject*: [mg127817] Getting the Derivative of an HoldForm Expression*From*: nitgun at gmail.com*Date*: Thu, 23 Aug 2012 20:52:35 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net

Hi, am currently fiddling with some equations. To make it more easy I want to introduce subexpressions which should not be evaluated. For this purpose I found HoldForm. But this creates some weird results when I want to get the derivative of such a term. But let's have a look at an example: In1:=T=HoldForm[2*a[t]] Out1=2 a[t] In2:= a[t]=b[t]+c[t] Out2=b[t]+c[t] In3:=T Out3=2 a[t] <-- this is what I need HoldForm for, as I do not want 2(b[t]+c[t]) as result In4:=D[T,t] Out4=2 a'[t] HoldForm'[2(b[t]+c[t])] I would expect 2 a'[t] as result so the term HoldForm'[...] seems to be a bit weird to me. Can anyone explain to me how I can avoid this term? Thanks in advance.