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A023815
Number of binary operations on an n-set that are commutative and associative; labeled commutative semigroups.
12
1, 1, 6, 63, 1140, 30730, 1185072, 66363206, 7150843144, 3829117403448
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OFFSET
0,3
LINKS
Table of n, a(n) for n=0..9.
Eric Postpischil
Posting to sci.math newsgroup, May 21 1990
Amit Sehgal, Sunil Kumar, Sarita, Yashpal,
Commutative Associative Binary Operations on a Set with Five Elements
, J. Phys.: Conf. Ser. (2018) 1000 012063
Eric Weisstein's World of Mathematics,
Semigroup.
Index entries for sequences related to semigroups
FORMULA
a(n) +
A079192
(n) +
A079195
(n) +
A079198
(n) =
A002489
(n).
a(n) = Sum_{k>=1}
A079201
(n,k)*
A079210
(n,k). -
Andrew Howroyd
, Jan 26 2022
CROSSREFS
Row sums of
A058167
.
Cf.
A001423
,
A001426
(isomorphism classes),
A023813
(commutative only),
A023814
(associative only),
A027851
.
Cf.
A002489
,
A079192
,
A079195
,
A079198
,
A079201
,
A079210
.
Sequence in context:
A271607
A360481
A079244
*
A249590
A034665
A339140
Adjacent sequences:
A023812
A023813
A023814
*
A023816
A023817
A023818
KEYWORD
nonn
,
hard
,
more
AUTHOR
Lyle Ramshaw (ramshaw(AT)pa.dec.com)
EXTENSIONS
a(8) from
Andrew Howroyd
, Jan 26 2022
a(9) from
Andrew Howroyd
, Feb 14 2022
STATUS
approved
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Last modified April 28 11:26 EDT 2024. Contains 372049 sequences. (Running on oeis4.)