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A308354
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Number of (2k+1)-ary quasitrivial semigroups that have two neutral elements on an n-element set.
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2
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0, 1, 3, 24, 200, 2070, 24822, 340648, 5257800, 90174690, 1701190370, 35011502460, 780603478668, 18742820292742, 482172697215510, 13231193297338320, 385766358723033104, 11908944548154971946, 388063941316923002634, 13310969922203225028580
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OFFSET
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1,3
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COMMENTS
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Number of (2k+1)-ary associative and quasitrivial operations that have two neutral elements on an n-element set.
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LINKS
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FORMULA
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a(n) = binomial(n,2)*A292932(n-2) for n >= 2.
a(n) ~ n! / (2*(r-1) * (r-3)^(n-1)), where r = -LambertW(-1, -2*exp(-3)). - Vaclav Kotesovec, Jun 05 2019
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MATHEMATICA
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nmax = 20; Rest[CoefficientList[Series[x^2/(3 - 2*E^x + x)/2, {x, 0, nmax}], x] * Range[0, nmax]!] (* Vaclav Kotesovec, Jun 05 2019 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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