a(1) = 1; a(n) = 1 + Sum_{i=1..n} Product_{j=i..2*i-1} (n-j).

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =11a(5) =24a(6) =51a(7) =122a(8) =291a(9) =756a(10) =1979a(11) =5526a(12) =15627a(13) =46496a(14) =140451a(15) =442194a(16) =1414931a(17) =4687212a(18) =15785451a(19) =54764846a(20) =193129659a(21) =698978136a(22) =2570480147a(23) =9672977706a(24) =36967490691a(25) =144232455524a(26) =571177352091