%I #10 Jun 13 2012 21:04:25
%S 1,6,159,332380,2751884514765,272622932796281408879065986,
%T 3641839910835401567626683593436003894250931310990279691
%N Number of elements in the free band (idempotent semigroup) on n generators.
%C An idempotent semigroup satisfies the equation xx=x for any element x.
%D J. Howie, Fundamentals of Semigroup Theory, Oxford University Press 1995, p. 123.
%H <a href="/index/Se#semigroups">Index entries for sequences related to semigroups</a>
%F a(n) = Sum_{k=1..n} C(n, k) A030450(k).
%Y Cf. A030450. A005345(n)=a(n)+1.
%K nonn
%O 1,2
%A marcel_j(AT)hilbert.maths.utas.edu.au (Marcel Jackson)