Pelletier, Francis Jeffry (2000). «Review of Metamathematics of fuzzy logics» (PDF). The Bulletin of Symbolic Logic. 6 (3): 342—346. JSTOR 421060. doi:10.2307/421060
Kumar S. Ray «Soft Computing and Its Applications, Volume One: A Unified Engineering Concept» // CRC Press, 2014, ISBN 1-926895-38-X, 9781926895383. P. 349 «As early as 1965, independent of Zadeh’s approach, the German mathematician Dieter Klaua presents two versions for a cumulative hierarchy of many-valued sets.»
