       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)
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```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?