a(1) = 1; a(n+1) = a(n) + Sum_{d|n} a(n/d) * a(d).

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =58a(5) =174a(6) =546a(7) =1638a(8) =4986a(9) =14994a(10) =45214a(11) =135642a(12) =407838a(13) =1223514a(14) =3672726a(15) =11018874a(16) =33063498a(17) =99190494a(18) =297593514a(19) =892780542a(20) =2678403690a(21) =8035217622a(22) =24105833722a(23) =72317501166a(24) =216953071986a(25) =650859219322