# The stiffness matrix

**URL:** <https://community.freefem.org/t/the-stiffness-matrix/1591>\
**Category:** General Discussion\
**Created:** [March 9, 2022, 2:33am UTC](https://community.freefem.org/t/the-stiffness-matrix/1591 "2022-03-09T02:33:59Z")\
**Posts on this page:** 2\
**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:** [March 9, 2022, 2:33am UTC](https://community.freefem.org/t/the-stiffness-matrix/1591/1 "2022-03-09T02:33:59Z")

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When calculating the spatial error and convergence order, the determinant of the stiffness matrix is non-zero, but the following error and convergence order are obtained

> L2error nan H1error nan  
> L2error nan H1error nan  
> L2error nan H1error nan  
> L2error nan H1error nan  
> L2error nan H1error nan  
> L2error nan H1error nan  
> CL2 nan CH1 nan  
> CL2 nan CH1 nan  
> CL2 nan CH1 nan  
> CL2 nan CH1 nan  
> CL2 nan CH1 nan

why?

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**Author:** ![SamuelJosephs](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/samueljosephs/32/1046_2.png) [@SamuelJosephs](https://community.freefem.org/u/SamuelJosephs)\
**Post date:** [March 9, 2022, 8:58pm UTC](https://community.freefem.org/t/the-stiffness-matrix/1591/2 "2022-03-09T20:58:45Z")

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If you post your code someone might be able to help you.