a(n) = A027113(n, 2n).

Terms

a(0) =1a(1) =2a(2) =3a(3) =7a(4) =16a(5) =39a(6) =98a(7) =252a(8) =659a(9) =1747a(10) =4682a(11) =12662a(12) =34505a(13) =94644a(14) =261069a(15) =723707a(16) =2014952a(17) =5631875a(18) =15796210a(19) =44444302a(20) =125405217a(21) =354765866a(22) =1006003375a(23) =2858939669a(24) =8141135778a(25) =23225997936a(26) =66376634429a(27) =190000938785