a(n) = Lucas(3*n) - 1.

Terms

a(0) =1a(1) =3a(2) =17a(3) =75a(4) =321a(5) =1363a(6) =5777a(7) =24475a(8) =103681a(9) =439203a(10) =1860497a(11) =7881195a(12) =33385281a(13) =141422323a(14) =599074577a(15) =2537720635a(16) =10749957121a(17) =45537549123a(18) =192900153617a(19) =817138163595