a(n) = 15 + floor((2 + Sum_{j=1..n-1} a(j))/3).

Terms

a(0) =15a(1) =20a(2) =27a(3) =36a(4) =48a(5) =64a(6) =85a(7) =114a(8) =152a(9) =202a(10) =270a(11) =360a(12) =480a(13) =640a(14) =853a(15) =1137a(16) =1516a(17) =2022a(18) =2696a(19) =3594a(20) =4792a(21) =6390a(22) =8520a(23) =11360a(24) =15146a(25) =20195a(26) =26927a(27) =35902a(28) =47870a(29) =63826