Finding Lyapunov functions I came across the following question in one of my professor's past exams: 
Find a Lyapunov function for $(0,0)$ in the system:
$$
\left\{
\begin{array}{ll}
\dot{x} = -x -2y + x^2\\
\dot{y} = x - 4y + xy
\end{array}\right.
$$
I know there is no formula for finding Lyapunov functions for a system, so how do I start solving such problems?
Thanks!

For many ODEs, a good bet is to try a polynomial as a Lyapunov function candidate. In fact, in practice, a common method is to search for a sum-of-squares polynomial, i.e., a polynomial that can be given as $\sum_{i=1}^k p_i(x)^2$, with $p_1, \dots, p_k$ polynomial.
I haven't tried this example, but a good approach might be to try something of the form $V(x,y) = (x+ay+b)^2 + cy^2 + d$, with parameters $a,b,c,d$.
