# How to use mean-zero pressure space in FreeFem

**URL:** <https://community.freefem.org/t/how-to-use-mean-zero-pressure-space-in-freefem/3570>\
**Category:** General Discussion\
**Created:** [November 2, 2024, 7:02am UTC](https://community.freefem.org/t/how-to-use-mean-zero-pressure-space-in-freefem/3570 "2024-11-02T07:02:22Z")\
**Posts on this page:** 1\
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**Author:** ![fb77](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/fb77/32/3796_2.png) [@fb77](https://community.freefem.org/u/fb77)\
**Post date:** [November 2, 2024, 10:56am UTC](https://community.freefem.org/t/how-to-use-mean-zero-pressure-space-in-freefem/3570/2 "2024-11-02T10:56:43Z")

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You have to use a constraint and a Lagrange multiplier,  
as in [Error convergence on Stokes problem](https://community.freefem.org/t/error-convergence-on-stokes-problem/3550)  
The Lagrange multiplier \lambda is a constant in space, and \mu is the associated test constant, that sets the constraint. To do it correctly you have to define a block matrix as in the FreeFem doc p.671 “Lagrange Multipliers”.

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