Proving strong lyapunov function I am working on the following problem:
Let $x'=-ax+bf(y)$ and $y'=cx-df(y)$ with $f(0)=0$, $yf(y)>0$ for $y \neq 0$ and $a,b,c,d>0$.  I need to show that the following function (for suitable values of p and q) is a strong Lyapunov function for the zero solutions of my system.
$$ V = px^2/2+q\int_{0}^{y} f(u) \,du  $$
I had no problem showing that $V>0$ for all nonzero input as it's easy to show that the integral is positive using the fact that $yf(y)>0$ and $f(0) = 0$
I then go on to try to show that the derivative of $V$ with respect to time is negative definite, which is where I run into problems.
$$\dot V = px\dot x + q\dot y f(y)$$
via the chain rule, but when I plug in the defined values of $\dot x$ and $\dot y$ I cannot figure out how to show that this expression is always less than zero.  Any advice as to how to start this process?
 A: From
$$\dot{V} = p x \dot{x} + q f(y) \dot{y},$$
the substitutions are made
$$\dot{V} = p x \left[- a x + b f(y)\right] + q f(y) \left[c x - d f(y)\right]$$
$$\dot{V} = - a p x^{2} + b p x f(y) + c q x f(y) - d q f^{2}(y)$$
and be rewritten in a compact form
$$\dot{V} = - \left[\matrix{x & f(y)}\right] \left[\matrix{a p & - b p \cr - c q & d q}\right] \left[\matrix{x \cr f(y)}\right]$$
$$\dot{V} = - \mathbf{x}^{T} \mathbf{P} \mathbf{x}.$$
To ensure that $\dot{V} < 0$ for $\mathbf{x} \neq \mathbf{0}$, then the matrix $\mathbf{P} = \left[\matrix{a p & - b p \cr - c q & d q}\right]$ has to be positive-definite.
To ensure that the matrix $\mathbf{P}$ is positive-definite, and this boils down to having the condition
$$a p d q - b p c q > 0$$
Edit: Since $\mathbf{P}$ has to be symmetric for the positive-definite property, the values for $p$ and $q$ can be selected to be $p = \frac{1}{b}$ and $q = \frac{1}{c}$ so that
$$\mathbf{P} = \left[\matrix{\frac{a}{b} & -1 \cr -1 & \frac{d}{c}}\right]$$
Then, the inequality becomes
$$\frac{ad}{bc} - 1 > 0$$
and if the condition is satisfied, all of the eigenvalues of $\mathbf{P}$ are positive. It is also possible to select other values for $p$ and $q$.
A: After insertion you get
\begin{align}
\dot V &= -pax^2 + (pb+qc)xf(y)-qdf(y)^2
\\
4pa\dot V&=-\Bigl(2pax-(pb+qc)f(y)\Bigr)^2+\Bigl[(pb+qc)^2-4paqd\Bigr]f(y)^2
\\
&=-\Bigl(2pax-(pb+qc)f(y)\Bigr)^2+\Bigl[(pb-qc)^2-4(ad-bc)pq\Bigr]f(y)^2
\end{align}
Now the aim is to have the coefficient of the second term be negative. The square can be reduced to zero using $p=c$, $q=b$. Then the sign condition only depends on if $ad-bc>0$.
In the case that the second coefficient is negative, slight variations of $p$ and $q$ will keep that quality, so the values used above are not unique.
