V. Wiktor Marek Zdzislaw Pawlak Information Storage and Retrieval Systems: Mathematical Foundations. 331-354 1976 1 Theor. Comput. Sci. 4 db/journals/tcs/tcs1.html#MarekP76 https://doi.org/10.1016/0304-3975(76)90077-3 https://www.wikidata.org/entity/Q127199409