Cholewa, Piotr W. “Duality Principle of W. Sierpiński in the Abstract Baire-Cathegory Theory”. Annales Mathematicae Silesianae 2 (September 30, 1986): 30–36. Accessed December 5, 2025. https://journals.us.edu.pl/index.php/AMSIL/article/view/14315.