# Error：requiring an unknown u for the problem

**URL:** <https://community.freefem.org/t/error-requiring-an-unknown-u-for-the-problem/1224>\
**Category:** General Discussion\
**Created:** [September 26, 2021, 2:03am UTC](https://community.freefem.org/t/error-requiring-an-unknown-u-for-the-problem/1224 "2021-09-26T02:03:56Z")\
**Posts on this page:** 5\
**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:** [September 26, 2021, 2:03am UTC](https://community.freefem.org/t/error-requiring-an-unknown-u-for-the-problem/1224/1 "2021-09-26T02:03:56Z")

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I have definite the problem and solve, the software said,  
 ![image](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/1X/2ffeeab09a21008b4bcfec06a864a2ca0da5dd41.png)  
I don’t know what is wrong.

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**Author:** ![Jamal](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/jamal/32/1527_2.png) [@Jamal](https://community.freefem.org/u/Jamal)\
**Post date:** [September 26, 2021, 4:35pm UTC](https://community.freefem.org/t/error-requiring-an-unknown-u-for-the-problem/1224/2 "2021-09-26T16:35:01Z")

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Look at Dirichlet condition  
On(… , u= …)  
I think you use an other variable  
On(…, ???)

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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 27, 2021, 1:20am UTC](https://community.freefem.org/t/error-requiring-an-unknown-u-for-the-problem/1224/3 "2021-09-27T01:20:32Z")

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![image](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/1X/7b6ba1c9aa28bcdb9a88b9d4f839d2eb3542dab8.png)  
you mean the solve (u1) and on (u1)? they match,right?

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**Author:** ![Jamal](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/jamal/32/1527_2.png) [@Jamal](https://community.freefem.org/u/Jamal)\
**Post date:** [September 27, 2021, 10:24am UTC](https://community.freefem.org/t/error-requiring-an-unknown-u-for-the-problem/1224/4 "2021-09-27T10:24:25Z")

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Can you send me the instruction of the line 248

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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 27, 2021, 10:44am UTC](https://community.freefem.org/t/error-requiring-an-unknown-u-for-the-problem/1224/5 "2021-09-27T10:44:06Z")

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After changing the boundary condition，it works. Thanks for your advice.