a(n) = a(n-1) + 6*a(n-3) + 2*a(n-4), with a(0) = a(1) = 1, a(2) = 3, a(3) = 6.

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =14a(5) =34a(6) =76a(7) =172a(8) =404a(9) =928a(10) =2112a(11) =4880a(12) =11256a(13) =25784a(14) =59288a(15) =136584a(16) =313800a(17) =721096a(18) =1659176a(19) =3815144a(20) =8769320a(21) =20166568a(22) =46375784a(23) =106621992a(24) =245160040a(25) =563747880a(26) =1296231400a(27) =2980435624a(28) =6853242984a(29) =15758127144