A323260(n)/2.

Terms

a(0) =0a(1) =1a(2) =3a(3) =12a(4) =48a(5) =195a(6) =791a(7) =3211a(8) =13031a(9) =52884a(10) =214614a(11) =870949a(12) =3534489a(13) =14343685a(14) =58209627a(15) =236226664a(16) =958656488a(17) =3890425619a(18) =15788149015a(19) =64071562799a(20) =260015607607