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• Katětov, Miroslav: Products of filters. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 9 (1968), issue 1, pp. 173-189
• Frolík, Zdeněk: On two problems of W. W. Comfort. (English). Commentationes Mathematicae Universitatis Carolinae, vol. 8 (1967), issue 1, pp. 139-144