# How to evaluate simplify an expression containing derivatives

I have an expression that is a derivative of order 2. I want to assign value to derivatives of orders lower than 2(e.g. the expression itself) which should cause the derivatives of higher order become zero and the expression will be simplified. I could not make Mathematica do this simplification. Does anyone know what should I do to simplify in this case?

Here is a simple example:

In: ww[t]=rr[t]''
Out: rr''[t]
In: rr[a]=0
Out: 0
In: ww[a]
Out:rr[t]''

After I give a value to the expression, I want the derivative expression to become zero.

• Hello Alex; I do not think your last question was closed because it is only missing plain text code. It was closed because it is not clear what you are asking. What does I want to assign value to derivatives of orders lower than n which should cause the derivatives of higher order become zero actually mean? Could you write an example using normal math of what you are trying to do? image is OK if you can't use Latex. Nov 24 '21 at 8:04
• ...The code you posted says nothing about the "order" of the derivative. It just shows second derivative there hardcoded. There is not even an n in it. That is why it is not clear what you mean. Nov 24 '21 at 8:08
• Thanks for your help @Nasser. I have edited my question. Nov 24 '21 at 8:14
• If I understand correctly, you want older output adapting to newer input? You can do this using "Dynamic". However, setting rr[a]==0 does not make rr''[a] == 0 and ww[a]==0 Nov 24 '21 at 8:57