Mitropolsky , Yu. A., and A. K. Lopatin. 1988. “Asymptotic Decomposition of Fully Integrable Pfaff Systems With Perturbation”. Ukrains’kyi Matematychnyi Zhurnal 40 (3): 349-56. https://umj.imath.kiev.ua/index.php/umj/article/view/9148.