Can be computed in polynomial time, but probably not "easily".
D. E. Knuth, The Art of Computer Programming, Volume 4, to appear as an exercise in Section 7.1.4.
Table of n, a(n) for n=1..8.
Sequence in context: A231808 A306063 A028372 * A230054 A303569 A156523
Adjacent sequences: A130675 A130676 A130677 * A130679 A130680 A130681
Don Knuth, Jun 29 2007