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A030449 Number of elements in the free band (idempotent semigroup) on n generators. 2
1, 6, 159, 332380, 2751884514765, 272622932796281408879065986, 3641839910835401567626683593436003894250931310990279691 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
An idempotent semigroup satisfies the equation xx=x for any element x.
REFERENCES
J. Howie, Fundamentals of Semigroup Theory, Oxford University Press 1995, p. 123.
LINKS
FORMULA
a(n) = Sum_{k=1..n} C(n, k) A030450(k).
CROSSREFS
Cf. A030450. A005345(n)=a(n)+1.
Sequence in context: A282564 A324231 A283728 * A364370 A120277 A241453
KEYWORD
nonn
AUTHOR
marcel_j(AT)hilbert.maths.utas.edu.au (Marcel Jackson)
STATUS
approved

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Last modified April 18 18:58 EDT 2024. Contains 371781 sequences. (Running on oeis4.)