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

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =40a(5) =128a(6) =400a(7) =1248a(8) =3904a(9) =12224a(10) =38272a(11) =119808a(12) =375040a(13) =1174016a(14) =3675136a(15) =11504640a(16) =36014080a(17) =112738304a(18) =352915456a(19) =1104764928a(20) =3458351104a(21) =10826006528a(22) =33889681408a(23) =106088103936a(24) =332097716224