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

Terms

a(0) =1a(1) =3a(2) =7a(3) =23a(4) =71a(5) =261a(6) =925a(7) =3455a(8) =12877a(9) =48693a(10) =184757a(11) =705713a(12) =2704157a(13) =10401527a(14) =40116677a(15) =155120975a(16) =601080391a(17) =2333619351a(18) =9075135301a(19) =35345312513a(20) =137846529751a(21) =538258059199