a(n) = 2*a(n-1) + 4*a(n-2) - 4*a(n-3) - 4*a(n-4).

Terms

a(0) =1a(1) =1a(2) =4a(3) =8a(4) =24a(5) =60a(6) =168a(7) =448a(8) =1232a(9) =3344a(10) =9152a(11) =24960a(12) =68224a(13) =186304a(14) =509056a(15) =1390592a(16) =3799296a(17) =10379520a(18) =28357632a(19) =77473792a(20) =211662848a(21) =578272256a(22) =1579870208a(23) =4316282880a(24) =11792306176a(25) =32217174016a(26) =88018960384a(27) =240472260608a(28) =656982441984