Let us start with the Pohozaev identity for semilinear elliptic equation with polygonal nonlinear terms of the form
over an open, star-shaped domain . We also assume is identical to zero on the boundary .
We multiply the PDE by and integrate over to find
.
The term on the left is just
Now
On the other hand, since on , is parallel to the normal at each point . Thus
.
Using this equality we calculate
.
Combining all gives
.
We now evaluate the right hand side.
Therefore the identity becomes
.
Definition. The following identity
is called the Pohozaev identity for the problem .
Note that, if is the star-shaped domain, we then obtain on . Consequently,
.
But once we multiply the PDE by and integrate by parts, we produce the equality
.
We thus conclude
.
Therefore, if , we need .
Many Thanks. This entry is nice. I have learned much.
Comment by HasPas — April 16, 2010 @ 21:49
Hi there,
First of all, thanks for these posts that help us so much. I was reading this articule and i think that in the third formula, second line (when you are using Green’s Theorem) its instead of .
Again Thanks!!
Comment by Urko — October 31, 2010 @ 7:11
Dear Urko,
Thank you for pointing out the misprint. You are right, the formula is nothing but integration by parts, that is
Comment by Ngô Quốc Anh — October 31, 2010 @ 10:39
In the calculation of seems to be a mistake with the signs. Since and the sign of is “-” then the formula following “Combining all gives” is wrong. Do you agree?
Comment by Urko — November 1, 2010 @ 3:56
Right 😉
Comment by Urko — October 31, 2010 @ 21:28
Hi Urko,
Thanks again. There is no doubt in both and . Concerning , there is no minus sign in fact, the correct formula should be
therefore there is no mistake in the Pohozaev indentity stated in the definition.
Thanks a lot for pointing out the mistake and also for your interest in my blog.
Comment by Ngô Quốc Anh — November 1, 2010 @ 12:17
If you have some difficulty, here is my interpretation
Comment by Ngô Quốc Anh — November 1, 2010 @ 12:26
Thanks Ngô,
I supposed that the mistake was in the sign of A_2 but i wasnt sure enough. Im trying to prove a Pohozaev-type identity for another equation (involving fractinal laplacians) and im using this proof as start point ;).
Thanks again.
Comment by Urko — November 1, 2010 @ 21:29