a(n) = (1/(2*n)) * Sum_{d|n} 3^(n/d-1) * phi(3*d).

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =17a(5) =44a(6) =105a(7) =278a(8) =733a(9) =1978a(10) =5369a(11) =14792a(12) =40881a(13) =113934a(14) =318884a(15) =896948a(16) =2532161a(17) =7174862a(18) =20390553a(19) =58114072a(20) =166037460a(21) =475473286a(22) =1364393897a(23) =3922640132a(24) =11297181473a(25) =32588043882a(26) =94143179560a(27) =272342824320a(28) =788854912241