The power of commuting with finite sets of words

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Authors	KUNC Michal
Year of publication	2005
Type	Article in Proceedings
Conference	STACS 2005: 22nd Annual Symposium on Theoretical Aspects of Computer Science, Stuttgart, Germany, February 24-26, 2005. Proceedings
MU Faculty or unit	Faculty of Science
Citation
Web	http://www.springerlink.com/link.asp?id=72ul1qvmpr3utqyp
Field	General mathematics
Keywords	Commutation of languages; Language equation; Regular language; Recursively enumerable language; Minsky machine
Description	We show that one can construct a finite language L such that the largest language commuting with L is not recursively enumerable. This gives a negative answer to the question raised by Conway in 1971 and also strongly disproves Conway's conjecture on context-freeness of maximal solutions of systems of semi-linear inequalities.
Related projects:	Mathematical structures of algebra and geometry