a(n) = Lucas(2*n+3) - (6*n+4).

Terms

a(0) =1a(1) =13a(2) =54a(3) =171a(4) =487a(5) =1324a(6) =3525a(7) =9297a(8) =24418a(9) =64015a(10) =167691a(11) =439128a(12) =1149769a(13) =3010261a(14) =7881102a(15) =20633139a(16) =54018415a(17) =141422212a(18) =370248333a(19) =969322905a(20) =2537720506a(21) =6643838743a(22) =17393795859a(23) =45537548976a(24) =119218851217a(25) =312119004829