# Applying several BC to a boundary

**URL:** <https://community.freefem.org/t/applying-several-bc-to-a-boundary/1940>\
**Category:** General Discussion\
**Created:** [August 12, 2022, 4:19am UTC](https://community.freefem.org/t/applying-several-bc-to-a-boundary/1940 "2022-08-12T04:19:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![mahdi](https://avatars.discourse-cdn.com/v4/letter/m/e9c0ed/32.png) [@mahdi](https://community.freefem.org/u/mahdi)\
**Post date:** [August 12, 2022, 4:19am UTC](https://community.freefem.org/t/applying-several-bc-to-a-boundary/1940/1 "2022-08-12T04:19:27Z")

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Hi there;

I wonder if anyone can help me find out how I can apply several derivatives in a boundary; in a 1D problem  
\frac{\partial \phi}{\partial x}=\alpha  
\frac{\partial^2 \phi}{\partial x^2}=\beta  
\frac{\partial^3 \phi}{\partial x^3}=\gamma  
How can I impose them to a single boundary?  
And does it make sense? If so, is there any consideration on the element type that should be used?

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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:** [August 12, 2022, 11:53am UTC](https://community.freefem.org/t/applying-several-bc-to-a-boundary/1940/2 "2022-08-12T11:53:27Z")

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You have to write the correct variationnal form of you problem, I thing it is a Tri-Laplacien , this imply C^2 conformity and we do not have is regularity on FreeFEM++ finite element.