# About syntax error

**URL:** <https://community.freefem.org/t/about-syntax-error/1182>\
**Category:** General Discussion\
**Created:** [August 31, 2021, 2:44am UTC](https://community.freefem.org/t/about-syntax-error/1182 "2021-08-31T02:44:28Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![xiaojiemo](https://avatars.discourse-cdn.com/v4/letter/x/a4c791/32.png) [@xiaojiemo](https://community.freefem.org/u/xiaojiemo)\
**Post date:** [August 31, 2021, 2:44am UTC](https://community.freefem.org/t/about-syntax-error/1182/1 "2021-08-31T02:44:28Z")

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I’m trying to write a problem about the equation like below,  
 ![image](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/1X/635362eaf4d6386378e8ae2c2a685f1ec7a373b5.png)  
and I wrote:  
problem displacementf(uf,duf)=int2d(th2)((dx(duf)\*dx(uf)+dy(duf)\*dy(uf))\*uf)-int2d(th2)((dx(duf)\*dx(uf)+dy(duf)\*dy(uf))\*convect([up1,up2],-dt,uf));  
the procedure said the \* is an wrong syntax.  
I don’t know why.

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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:** [September 13, 2021, 11:29am UTC](https://community.freefem.org/t/about-syntax-error/1182/2 "2021-09-13T11:29:21Z")

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is it a weak form of some pde?  
also, what is duf and ((uf)\_s)^’ ?

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<div class="post-metadata">

**Author:** ![xiaojiemo](https://avatars.discourse-cdn.com/v4/letter/x/a4c791/32.png) [@xiaojiemo](https://community.freefem.org/u/xiaojiemo)\
**Post date:** [September 16, 2021, 2:27am UTC](https://community.freefem.org/t/about-syntax-error/1182/3 "2021-09-16T02:27:28Z")

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yes,it is a weak form. duf is the test function and equal to the partial(uf). (uf)'s=partial(uf)/partial(t)+vs\*grad(uf). vs is the velocity.

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<div class="post-metadata">

**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:** [September 16, 2021, 6:10am UTC](https://community.freefem.org/t/about-syntax-error/1182/4 "2021-09-16T06:10:06Z")

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If I am not wrong:

your governing PDEs is

uf\_t + vs . ( grad(uf) ) + grad( uf ) \* [0, wf] - vf = 0

if Yes, then you need to linearized your problem first then you can solve it.

Also, if possible try to give the cartesian form of governing equation without weak formulation.

it is given here.  
[https://doc.freefem.org/tutorials/navierStokesNewton.html](https://doc.freefem.org/tutorials/navierStokesNewton.html)