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A023815 Number of binary operations on an n-set that are commutative and associative; labeled commutative semigroups. 10
1, 1, 6, 63, 1140, 30730, 1185072, 66363206 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Table of n, a(n) for n=0..7.

Eric Postpischil Posting to sci.math newsgroup, May 21 1990

Eric Weisstein's World of Mathematics, Semigroup.

Index entries for sequences related to semigroups

CROSSREFS

Cf. A001423, A001426, A023813, A023814, A027851.

a(n)+A079192(n)+A079195(n)+A079198(n)=A002489(n)

Sequence in context: A280476 A271607 A079244 * A249590 A034665 A218383

Adjacent sequences:  A023812 A023813 A023814 * A023816 A023817 A023818

KEYWORD

nonn

AUTHOR

Lyle Ramshaw (ramshaw(AT)pa.dec.com)

STATUS

approved

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Last modified August 19 19:25 EDT 2017. Contains 290821 sequences.