# Fourth order derivative Help

**URL:** <https://community.freefem.org/t/fourth-order-derivative-help/1150>\
**Category:** General Discussion\
**Created:** [August 11, 2021, 7:59am UTC](https://community.freefem.org/t/fourth-order-derivative-help/1150 "2021-08-11T07:59:04Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![sumantkr](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/sumantkr/32/2681_2.png) [@sumantkr](https://community.freefem.org/u/sumantkr)\
**Post date:** [August 11, 2021, 7:59am UTC](https://community.freefem.org/t/fourth-order-derivative-help/1150/1 "2021-08-11T07:59:04Z")

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Hello Everyone

error message appearing for third or fourth order derivatives.

e.g. for u\_xxx I have written `dxxx(u)` but it is showing identifier does not exist.

Help me to fix it.