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A118100
Number of commutative semigroups of order <= n.
0
1, 2, 5, 17, 75, 400, 2543, 19834, 241639
OFFSET
0,2
COMMENTS
A001426(n) is the number of commutative semigroups of order n. A001426(n) + A079193(n) + A079196(n) + A079199(n) = A001329(n). 2, 5, 17, 2543 and 241639 are primes.
FORMULA
a(n) = SUM[i=1..n] A001426(i).
EXAMPLE
a(8) = 1 + 1 + 3 + 12 + 58 + 325 + 2143 + 17291 + 221805 = 241639.
CROSSREFS
KEYWORD
nonn
AUTHOR
Jonathan Vos Post, May 11 2006
STATUS
approved