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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

Table of n, a(n) for n=1..7.

Index entries for sequences related to semigroups

FORMULA

a(n) = Sum_{k=1..n} C(n, k) A030450(k).

CROSSREFS

Cf. A030450. A005345(n)=a(n)+1.

Sequence in context: A237529 A282564 A283728 * A120277 A241453 A193370

Adjacent sequences:  A030446 A030447 A030448 * A030450 A030451 A030452

KEYWORD

nonn

AUTHOR

marcel_j(AT)hilbert.maths.utas.edu.au (Marcel Jackson)

STATUS

approved

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Last modified February 23 21:23 EST 2018. Contains 299588 sequences. (Running on oeis4.)