# [deal.II] Shape function derivative wrt to a different cell

```Hello all,

I am using deal.ii to solve a 2d solid mechanics problem.```
```
I want to do the following thing: I have a scalar quantity available
only at the Gauss points of a cell. (cell = 2d rectangular element)
I want to get the derivative of this quantity at the center of the cell.

One way to do this is to locally project the gauss point variable to the
nodes
and then take the derivative.

What I want to do is the following: Think of the 4 gauss points as a new
rectangular element.
Then, these 4 vertices become the node and this makes the value of the
variable known at the nodes of this element.

Now, I want to take the derivative of the variable wrt to this new cell
(vertices of new cell = gauss points of parent cell).

this new cell?

Some thought: Mapping eulerian may not work here as that mapping applies to
the nodes and is continuous. Here, the same node gets mapped to different
vertices (gauss point of the cell of which the node is a part of), so a
single mapping may not work. Either I use different Euler vector for the
different element but will that be expensive?

Thanks.

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