a(n) = 2^(n-2)*(n^2 - n + 4).

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =64a(5) =192a(6) =544a(7) =1472a(8) =3840a(9) =9728a(10) =24064a(11) =58368a(12) =139264a(13) =327680a(14) =761856a(15) =1753088a(16) =3997696a(17) =9043968a(18) =20316160a(19) =45350912a(20) =100663296a(21) =222298112a(22) =488636416a(23) =1069547520a(24) =2332033024a(25) =5066719232a(26) =10972299264a(27) =23689428992