# Approximation of the second order derivative

**URL:** <https://community.freefem.org/t/approximation-of-the-second-order-derivative/1465>\
**Category:** General Discussion\
**Created:** [February 1, 2022, 3:34pm UTC](https://community.freefem.org/t/approximation-of-the-second-order-derivative/1465 "2022-02-01T15:34:08Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![mariancantal](https://avatars.discourse-cdn.com/v4/letter/m/a9adbd/32.png) [@mariancantal](https://community.freefem.org/u/mariancantal)\
**Post date:** [February 1, 2022, 3:34pm UTC](https://community.freefem.org/t/approximation-of-the-second-order-derivative/1465/1 "2022-02-01T15:34:08Z")

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Hi!

I am trying to approximate the second-order derivative using dxx(). But this gives me zero values. P1 and P2 seemed to be not working. I tried using P3 but it gave me an unstable solution. Please help me. Thank you!

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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:** [February 3, 2022, 12:38pm UTC](https://community.freefem.org/t/approximation-of-the-second-order-derivative/1465/2 "2022-02-03T12:38:08Z")

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Remember of P1 finite element function the true dxx derivable is zero on each triangle + some jump of  
\frac{\partial u}{\partial {n}} on edge. An to get a correct convergence of dxx you need lot of regularity.