The power of commuting with finite sets of words

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Authors	KUNC Michal
Year of publication	2007
Type	Article in Periodical
Magazine / Source	Theory of Computing Systems
MU Faculty or unit	Faculty of Science
Citation
Web	http://dx.doi.org/10.1007/s00224-006-1321-z
Field	General mathematics
Keywords	Commutation of languages; Language equation; Regular language; Recursively enumerable language; Minsky machine
Description	We 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 and their physical applications