A030449 - OEIS

A030449

Number of elements in the free band (idempotent semigroup) on n generators.

1, 6, 159, 332380, 2751884514765, 272622932796281408879065986, 3641839910835401567626683593436003894250931310990279691

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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.

KEYWORD

nonn

AUTHOR

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

STATUS

approved