a(n) = s(n+3)/3, where s is A024737.

Terms

a(0) =1a(1) =3a(2) =9a(3) =28a(4) =87a(5) =270a(6) =838a(7) =2682a(8) =8568a(9) =27325a(10) =87087a(11) =284679a(12) =928180a(13) =3019131a(14) =9813285a(15) =32550310a(16) =107699892a(17) =355521969a(18) =1172334682a(19) =3930469221a(20) =13143936366a(21) =43850286550a(22) =146151275976a(23) =493751421315