Series-parallel numbers.

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =58a(5) =186a(6) =614a(7) =2034a(8) =6818a(9) =22970a(10) =77858a(11) =264970a(12) =905294a(13) =3102434a(14) =10661370a(15) =36722642a(16) =126752218a(17) =438294018a(18) =1518032598a(19) =5265341314a(20) =18286911130a(21) =63586988434a(22) =221342104842a(23) =771235606050