CONTEXT-FREE LANGUAGES, GROUPS, THE THEORY OF ENDS, SECOND-ORDER LOGIC, TILING PROBLEMS, CELLULAR AUTOMATA, AND VECTOR ADDITION SYSTEMSMULLER DE; SCHUPP PE.1981; BULL. AM. MATH. SOC.; ISSN 0002-9904; USA; DA. 1981; VOL. 4; NO 3; PP. 331-334; BIBL. 4 REF.Article

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