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

Terms

a(0) =0a(1) =1a(2) =2a(3) =8a(4) =20a(5) =60a(6) =160a(7) =448a(8) =1216a(9) =3344a(10) =9120a(11) =24960a(12) =68160a(13) =186304a(14) =508928a(15) =1390592a(16) =3799040a(17) =10379520a(18) =28357120a(19) =77473792a(20) =211661824a(21) =578272256a(22) =1579868160a(23) =4316282880a(24) =11792302080a(25) =32217174016a(26) =88018952192a(27) =240472260608