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 A293005 Number of associative, quasitrivial, and order-preserving binary operations on the n-element set {1,...,n}. 4
 0, 1, 4, 12, 34, 94, 258, 706, 1930, 5274, 14410, 39370, 107562, 293866, 802858, 2193450, 5992618, 16372138, 44729514, 122203306, 333865642, 912137898, 2492007082, 6808289962, 18600594090, 50817768106, 138836724394, 379308985002, 1036291418794, 2831200807594 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Colin Barker, Table of n, a(n) for n = 0..1000 M. Couceiro, J. Devillet, and J.-L. Marichal, Quasitrivial semigroups: characterizations and enumerations, arXiv:1709.09162 [math.RA] (2017). Jimmy Devillet, Bisymmetric and quasitrivial operations: characterizations and enumerations, arXiv:1712.07856 [math.RA], 2017. Index entries for linear recurrences with constant coefficients, signature (3,0,-2). FORMULA G.f.: x(x+1) / (2x^3-3*x+1). a(0) = 0, a(1) = 1, a(n+2)-2*a(n+1)-2*a(n) = 2. 3*a(n)+2 = Sum_{k>=0} (2*binomial(n,2*k)+3*binomial(n,2*k+1))*3^k. From Colin Barker, Sep 28 2017: (Start) a(n) = (-4 - (1-sqrt(3))^n*(-2+sqrt(3)) + (1+sqrt(3))^n*(2+sqrt(3))) / 6. a(n) = 3*a(n-1) - 2*a(n-2) for n>2. (End) PROG (PARI) concat(0, Vec(x*(1 + x) / ((1 - x)*(1 - 2*x - 2*x^2)) + O(x^30))) \\ Colin Barker, Sep 28 2017 CROSSREFS Cf. A002605, A028860, A293006, A293007. Sequence in context: A176753 A248873 A180224 * A173412 A079818 A115390 Adjacent sequences:  A293002 A293003 A293004 * A293006 A293007 A293008 KEYWORD nonn,easy AUTHOR J. Devillet, Sep 28 2017 STATUS approved

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Last modified May 19 23:40 EDT 2022. Contains 353847 sequences. (Running on oeis4.)