Second partial sums of A001891.

Terms

a(0) =1a(1) =6a(2) =21a(3) =57a(4) =133a(5) =281a(6) =554a(7) =1039a(8) =1878a(9) =3302a(10) =5686a(11) =9638a(12) =16143a(13) =26796a(14) =44179a(15) =72471a(16) =118435a(17) =193015a(18) =313920a(19) =509805a(20) =827036a(21) =1340636a(22) =2171996a(23) =3517532a(24) =5695053a(25) =9218786a(26) =14920769a(27) =24147269a(28) =39076593a(29) =63233317