Bondar’, O. P. “Generalized (CO)Homology Length of a Manifold and Functions With Singular Submanifolds”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 6, June 1996, pp. 843-6, https://umj.imath.kiev.ua/index.php/umj/article/view/5284.