# Getting problem to compute order of convergence in Poisson eqn when exact solution in unknown
**URL:** https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306
**Category:** Feature Request
**Created:** [June 12, 2024, 6:07am UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306 "2024-06-12T06:07:26Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Monirul25](https://community.freefem.org/u/Monirul25)
#### Post date: [June 12, 2024, 6:07am UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306/1 "2024-06-12T06:07:26Z")
Here, i am sharing my codes. Please help if anyone know. Moreover, I want to do simulation on this codes. How to do simulation on this code so that i can see my solution how simulate at different time??.
[poisson.edp](https://community.freefem.org/uploads/short-url/mW9OXhwEjq7fwYZMf2TStVEz5Q0.edp) (1.7 KB)
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### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [June 12, 2024, 8:08am UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306/2 "2024-06-12T08:08:10Z")
I sorry , but I do not understand what you want . You have no time in your equation so different time is no clean.
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### Author:  [@Monirul25](https://community.freefem.org/u/Monirul25)
#### Post date: [June 12, 2024, 8:23am UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306/3 "2024-06-12T08:23:25Z")
But please tell how i ger different L2-error so that i can compute order of convergence. I am getting same L2-error in every mesh size. Solution is uknown i want to compute L2-error and order of convergence. Please tell that i part. Thanks
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### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [June 12, 2024, 9:48am UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306/4 "2024-06-12T09:48:36Z")
No problem, because in you cas we know de exact solution
the corrected script:
[poisson.edp](https://community.freefem.org/uploads/short-url/7umO1CAM0RRI6PfRq56zZUnftmd.edp) (1.9 KB)
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### Author:  [@Monirul25](https://community.freefem.org/u/Monirul25)
#### Post date: [June 12, 2024, 11:22am UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306/5 "2024-06-12T11:22:56Z")
real ccc= 2./(2_pi^2);
func uexact = ccc_sin(pi_x)sin(piy);
func Lapuexact= - ccc_2_pi^2_sin(pi\*x)_sin(pi_y);
Sir, why you using this, i don’t know eaxct solution then i want compute L2-error and order of convergence. That is, **i want to compute L^2-error and order of convergence without using exact solution.**
**Suppose i don’t know the exact solution then, i want to compute L^2-error and order of convergence .** Infact in lastly, you are setting uhold=ueaxct Which is eaxctly means i know eaxct solution. But this i not want sir. I want that i know only f and u=0 boundary then compute L^2-error and order of convergence.
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### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [June 12, 2024, 2:47pm UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306/6 "2024-06-12T14:47:53Z")
I sorry, I you do no know the exact solution you car try to computation a good approximation on very very fine mesh, and use this solution as like an exact solution.
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### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [June 12, 2024, 3:06pm UTC](https://community.freefem.org/t/getting-problem-to-compute-order-of-convergence-in-poisson-eqn-when-exact-solution-in-unknown/3306/7 "2024-06-12T15:06:06Z")
My way:
[poisson-comp-fine-sol.edp](https://community.freefem.org/uploads/short-url/2cExelCj4dqSnkpsErXJJeVF2QA.edp) (1.0 KB)