(1/3)*s(n+1), where s = A026222.

Terms

a(0) =1a(1) =3a(2) =5a(3) =8a(4) =9a(5) =11a(6) =14a(7) =15a(8) =17a(9) =20a(10) =23a(11) =24a(12) =26a(13) =27a(14) =29a(15) =32a(16) =33a(17) =35a(18) =38a(19) =41a(20) =42a(21) =44a(22) =45a(23) =47a(24) =50a(25) =51a(26) =53a(27) =56a(28) =59a(29) =60