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[Question #107819]: Is my weak form right?

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From: cutejeff <question107819@xxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 19 Apr 2010 06:36:16 -0000
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New question #107819 on DOLFIN:
https://answers.launchpad.net/dolfin/+question/107819

To solve nonlinear PDE 

du/dt+3*lmbda*u^2*du/dx*(sina-cosa*du/dx)-lmbda*u^3*cosa*d2u/dx2=0

is the following weak form right? 

L = v*u*dx-v*u0*dx+dt*lmbda*v*3*u*u*u.dx(0)*(sin(alpha)-cos(alpha)*u.dx(0))*dx+dt*lmbda*u*u*u*cos(alpha)*u.dx(0)*v.dx(0)*dx
a = derivative(L, u, du)

I am sure the rest code for newton's method and time steps should be right. But I got the wrong result. I notice that there is a 
(du/dx)^2 in the 2nd term. I guess it may cause some problems. Because if I get rid of the (du/dx)^2, the result is exactly same with my finite difference code (which should be right), If I keep it there, it's different.

Anyone know why? Thank you.





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