Number of different groupings among the hierarchical orderings of n unlabeled elements.

Terms

a(0) =1a(1) =4a(2) =11a(3) =35a(4) =98a(5) =294a(6) =832a(7) =2401a(8) =6774a(9) =19137a(10) =53466a(11) =148994a(12) =412233a(13) =1136383a(14) =3116654a(15) =8515706a(16) =23172455a(17) =62836916a(18) =169801824a(19) =457406173a(20) =1228382159a(21) =3289493887a(22) =8784935160a(23) =23400668297a(24) =62179339101a(25) =164832960183a(26) =435978612329