a(n) = A026618(2*n, n-2).

Terms

a(0) =1a(1) =11a(2) =50a(3) =212a(4) =870a(5) =3509a(6) =14014a(7) =55640a(8) =220116a(9) =868870a(10) =3425092a(11) =13491064a(12) =53117350a(13) =209097945a(14) =823111350a(15) =3240499440a(16) =12759776700a(17) =50254414650a(18) =197979380220a(19) =780170359800