A323265(n)/2.

Terms

a(0) =0a(1) =1a(2) =3a(3) =13a(4) =51a(5) =208a(6) =842a(7) =3419a(8) =13873a(9) =56303a(10) =228487a(11) =927252a(12) =3762976a(13) =15270937a(14) =61972603a(15) =251497601a(16) =1020629091a(17) =4141923220a(18) =16808778106a(19) =68213486019a(20) =276824385713