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 A180043 The number of isomorphism classes of Szasz (uniquely non-associative) groupoids of order n. 0
 0, 0, 10, 24, 2064, 39961, 1194828 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A Szasz groupoid (S,*) is one for which there is exactly one ordered triple (a,b,c) of members of S that does not associate:  (a*b)*c != a*(b*c).  For any other triple (x,y,z), we have (x*y)*z = x*(y*z).  Thus, a Szasz groupoid is as close to being a semigroup as possible, without actually being associative.  G. Szasz proved that such groupoids exist on any set with at least four members.  Every Szasz groupoid is non-commutative. REFERENCES G. Szasz, Die Unabhangigkeit der Assoziativitatsbedingungen, Acta. Sci. Math. Szeged 15 (1953), 20-28. LINKS EXAMPLE The "smallest" Szasz groupoid of order 3 with elements {a,b,c} defines c*b = b, and the product of every other pair of elements is defined to be a. Then, (c*c)*b = a*b = b but c*(c*b) = c*b = b, but every triple other than (c,c,b) associates. CROSSREFS Sequence in context: A264296 A265150 A057462 * A251317 A251201 A048195 Adjacent sequences:  A180040 A180041 A180042 * A180044 A180045 A180046 KEYWORD nonn,hard,more AUTHOR James McCarron, Jan 14 2011 STATUS approved

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Last modified September 24 23:03 EDT 2020. Contains 337325 sequences. (Running on oeis4.)