# Euqiry about the interpolation in FreeFEM

**URL:** https://community.freefem.org/t/euqiry-about-the-interpolation-in-freefem/994
**Category:** General Discussion
**Created:** [May 26, 2021, 4:08am UTC](https://community.freefem.org/t/euqiry-about-the-interpolation-in-freefem/994 "2021-05-26T04:08:25Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![ligax](https://avatars.discourse-cdn.com/v4/letter/l/dec6dc/32.png) [@ligax](https://community.freefem.org/u/ligax)
#### Post date: [May 26, 2021, 4:08am UTC](https://community.freefem.org/t/euqiry-about-the-interpolation-in-freefem/994/1 "2021-05-26T04:08:25Z")

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Dear all,  
I meet some problems on interpolation.  
The following is the code:

solve LP(w,v)=  
int2d(V1)(dx(w)\*dx(v)+dy(w)\*dy(v))  
+int2d(V1)(dx(u2)\*dx(v)\*Xi+dy(u2)\*dy(v)\*Xi  
+dx(u2)\*v\*dx(Xi)+dy(u2)\*v\*dy(Xi))

Here w,v are in the same FEM space V1, while Xi is in another FEM space V2.  
Now I want to change the second integration to get this formula:  
int2d(V1)(dx(u2)\*dx(Ph(v\*Xi))+dy(u2)\*dy(Ph(v\*Xi)), where Ph is the interpolation operator in V1 space. Since v and Xi are FEM functions in different FEM spaces, would you please tell me how to implement this formula in code? Thank you so much!