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%I #22 Feb 17 2022 16:04:54
%S 1,1,6,63,1140,30730,1185072,66363206,7150843144,3829117403448
%N Number of binary operations on an n-set that are commutative and associative; labeled commutative semigroups.
%H Eric Postpischil <a href="http://groups.google.com/groups?&hl=en&lr=&ie=UTF-8&selm=11802%40shlump.nac.dec.com&rnum=2">Posting to sci.math newsgroup, May 21 1990</a>
%H Amit Sehgal, Sunil Kumar, Sarita, Yashpal, <a href="https://doi.org/10.1088/1742-6596/1000/1/012063">Commutative Associative Binary Operations on a Set with Five Elements</a>, J. Phys.: Conf. Ser. (2018) 1000 012063
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Semigroup.html">Semigroup.</a>
%H <a href="/index/Se#semigroups">Index entries for sequences related to semigroups</a>
%F a(n) + A079192(n) + A079195(n) + A079198(n) = A002489(n).
%F a(n) = Sum_{k>=1} A079201(n,k)*A079210(n,k). - _Andrew Howroyd_, Jan 26 2022
%Y Row sums of A058167.
%Y Cf. A001423, A001426 (isomorphism classes), A023813 (commutative only), A023814 (associative only), A027851.
%Y Cf. A002489, A079192, A079195, A079198, A079201, A079210.
%K nonn,hard,more
%O 0,3
%A Lyle Ramshaw (ramshaw(AT)pa.dec.com)
%E a(8) from _Andrew Howroyd_, Jan 26 2022
%E a(9) from _Andrew Howroyd_, Feb 14 2022
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