a(n) = ((4 + sqrt(7))^n + (4 - sqrt(7))^n)/2.

Terms

a(0) =1a(1) =4a(2) =23a(3) =148a(4) =977a(5) =6484a(6) =43079a(7) =286276a(8) =1902497a(9) =12643492a(10) =84025463a(11) =558412276a(12) =3711069041a(13) =24662841844a(14) =163903113383