a(n) = Sum_{d|n} binomial(n+d-2, n-1).

Terms

a(0) =1a(1) =3a(2) =7a(3) =25a(4) =71a(5) =280a(6) =925a(7) =3561a(8) =12916a(9) =49346a(10) =184757a(11) =710255a(12) =2704157a(13) =10427747a(14) =40119781a(15) =155288897a(16) =601080391a(17) =2334714319a(18) =9075135301a(19) =35352181116a(20) =137846759282a(21) =538302226628