A030449 Number of elements in the free band (idempotent semigroup) on n generators.

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

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: A282564 A324231 A283728 * A364370 A120277 A241453

Adjacent sequences: A030446 A030447 A030448 * A030450 A030451 A030452

Keyword

nonn

Author

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

Status

approved