Musaev, D. K. 2003. “Tubular (weakly) Π-Complete and (weakly) Π-Completeness of Continuous Mappings, Their Properties and Characterizations by the Use of Morphisms”. Methods of Functional Analysis and Topology 9 (3): 241-46. https://zen.imath.kiev.ua/index.php/mfat/article/view/218.