# Resolve two pde with FF++

**URL:** https://community.freefem.org/t/resolve-two-pde-with-ff/599
**Category:** General Discussion
**Created:** [October 4, 2020, 7:28pm UTC](https://community.freefem.org/t/resolve-two-pde-with-ff/599 "2020-10-04T19:28:15Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![Aicha](https://avatars.discourse-cdn.com/v4/letter/a/ec9cab/32.png) [@Aicha](https://community.freefem.org/u/Aicha)
#### Post date: [October 4, 2020, 7:28pm UTC](https://community.freefem.org/t/resolve-two-pde-with-ff/599/1 "2020-10-04T19:28:15Z")

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Hi,  
i havethe two following problems:

 ![problems](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/1X/43b17f5eb89d3fed41036803b90147096f372a07.png)  
(we add periodic boundary conditions to the two problems.  
I have the following code FF++ to calculate the approximate solutions of two problems and compare the curves of solutions[reg.edp](https://community.freefem.org/uploads/short-url/l2Wcn47O7p4fKZPCQPDPGTrzny2.edp) (3.3 KB)  
The problem is that the derivative of solutions are note periodic. I don’t understand the reason. Please can any one check the FF++ code and tell me if there is an error?  
Thank you in advance.

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### Author: ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [October 5, 2020, 2:05pm UTC](https://community.freefem.org/t/resolve-two-pde-with-ff/599/2 "2020-10-05T14:05:51Z")

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Remark,

your solution is periodic and the derivative are also periodic :

see your script after a small modification

solve square:mesh Th=square(N,N,[x+0.2,y]);  
add with a more graphics.  
[reg.edp](https://community.freefem.org/uploads/short-url/tjSsd3627E6ejPHOhDN4hwaswAb.edp) (3.4 KB)

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### Author: ![Aicha](https://avatars.discourse-cdn.com/v4/letter/a/ec9cab/32.png) [@Aicha](https://community.freefem.org/u/Aicha)
#### Post date: [October 5, 2020, 6:03pm UTC](https://community.freefem.org/t/resolve-two-pde-with-ff/599/3 "2020-10-05T18:03:42Z")

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Thank you so much Frédéric Hecht