What is the the expression in the (simple) Bendixson-Dulac theorem for a conservative system $x' = f, y' = g$, that is a system arising from a conservative second order equation $x'' + h(x) = 0$.

$\frac{\partial f}{\partial x} + \frac{\partial g}{\partial y} = {}$

Does the Bendixson-Dulac theorem apply? ? Yes No None of the above