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3. Universal Algebra and Applications in Theoretical Computer Science;Denecke,2002
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5. Y. Gao, Z. Esik, G. Liu, S. Yu, State complexity of intersection, union and catenation on k regular languages, Technical Report 715, Department of Computer Science, the University of Western Ontario, 2008 (Available from http://www.csd.uwo.ca/faculty/syu/public/TR3.pdf)