Hilbert's 16th problem. II. Pfaffian equations and variational methods

Starting from a Pfaffian equation in dimension $N$ and focusing on compact solutions for it, we place in perspective the variational method used in [29] to solve Hilbert's 16th problem. In addition to exploring how this viewpoint can help in detecting and finding approximations for limit cycles of planar systems, we recall some of the initial important facts of the full program developed in [29] to motivate that the same proposal could eventually be used in other situations. In particular, we make some initial interesting calculations in dimension $N=3$ that lead to some similar initial conclusions as with the case $N=2$.