a(n) = 2^(n-3)*(3*binomial(n,4) + 4*binomial(n,2) + 8).

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =70a(5) =252a(6) =904a(7) =3152a(8) =10560a(9) =33920a(10) =104704a(11) =311808a(12) =899584a(13) =2524160a(14) =6912000a(15) =18526208a(16) =48726016a(17) =126025728a(18) =321126400a(19) =807403520a(20) =2005794816a(21) =4929093632a(22) =11994136576a(23) =28924968960a(24) =69185044480a(25) =164240556032a(26) =387201368064a(27) =907009851392