# A nonlinear problem

**URL:** <https://community.freefem.org/t/a-nonlinear-problem/980>\
**Category:** General Discussion\
**Created:** [May 15, 2021, 8:45pm UTC](https://community.freefem.org/t/a-nonlinear-problem/980 "2021-05-15T20:45:44Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![fem-naive](https://avatars.discourse-cdn.com/v4/letter/f/7bcc69/32.png) [@fem-naive](https://community.freefem.org/u/fem-naive)\
**Post date:** [May 15, 2021, 8:45pm UTC](https://community.freefem.org/t/a-nonlinear-problem/980/1 "2021-05-15T20:45:44Z")

</div>

Hello

I am a beginner of FreeFem.  
I am studying how to solve nonlinear problems and make codes.  
In reference to the examples provided by the FREEFEM guidebook, I am trying to use the Newton method.

My problem is f= - div(|grad(u)|^2 \* grad(u)) .  
My goal is to make codes for FEM and DGM.  
I think that the code for FEM I wrote is fine but the code for DGM has problems.  
Probably, I do not understand how to differentiate weak formulae.

I have an error on the line below.  
intalledges(Th) ( mode\*( jump(u)_mean(2_(dx(u)\*dx(du)+dy(u)\*dy(du))\*dn(v)) )/nTonEdge )

Could you tell me why it causes an error. I do not understand the error message.

Thank you