a(n) = a(n-1) + 16*a(n-2), starting a(0)=1, a(1)=4.

Terms

a(0) =1a(1) =4a(2) =20a(3) =84a(4) =404a(5) =1748a(6) =8212a(7) =36180a(8) =167572a(9) =746452a(10) =3427604a(11) =15370836a(12) =70212500a(13) =316145876a(14) =1439545876a(15) =6497879892a(16) =29530613908a(17) =133496692180a(18) =605986514708