# The solution would be unstable,dxx(v)

**URL:** https://community.freefem.org/t/the-solution-would-be-unstable-dxx-v/1857
**Category:** General Discussion
**Created:** [June 29, 2022, 11:57am UTC](https://community.freefem.org/t/the-solution-would-be-unstable-dxx-v/1857 "2022-06-29T11:57:30Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![Qianrui](https://avatars.discourse-cdn.com/v4/letter/q/9fc348/32.png) [@Qianrui](https://community.freefem.org/u/Qianrui)
#### Post date: [June 29, 2022, 11:57am UTC](https://community.freefem.org/t/the-solution-would-be-unstable-dxx-v/1857/1 "2022-06-29T11:57:30Z")

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Hello everyone, I tried to write a small code about KdV. Because there is dxx (v), I used P3 finite element method, but the solution would be unstable. Why?

 ![屏幕截图 2022-06-29 195648](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/2X/5/54ae1229735f68df3da97477694842e64704cfdb.png)

[KDV(n=1)exact.edp](https://community.freefem.org/uploads/short-url/1llWQOf2DtAQeNTDM131X9cVRGE.edp) (1.2 KB)