I need to find a Energy function (Lyapunov function) for the next nonlinear coupled ODE system
$u'=-u+u^2v\\ v'=b-u^2v$
where $b\in\mathbb{R}^{+}$. The objective of this is to build a proper trapping region, but I can't fugure it out how to do it. I just don't know how to biuld a proper trapping region.
Then, once I got the trapping region, I'll apply the Poincaré-Bendixon theorem in order to prove that there's a limit cycle.