First column of a triangle - see Comments lines.

Terms

a(0) =1a(1) =2a(2) =3a(3) =7a(4) =17a(5) =40a(6) =93a(7) =216a(8) =502a(9) =1167a(10) =2713a(11) =6307a(12) =14662a(13) =34085a(14) =79238a(15) =184206a(16) =428227a(17) =995507a(18) =2314273a(19) =5380032a(20) =12507057a(21) =29075380a(22) =67592058a(23) =157132471a(24) =365288677a(25) =849193147a(26) =1974134558a(27) =4589306057a(28) =10668842202