(1/3)*s(n), where s(n) is the n-th multiple of 3 in A026142.

Terms

a(0) =1a(1) =4a(2) =2a(3) =8a(4) =3a(5) =12a(6) =14a(7) =5a(8) =6a(9) =20a(10) =7a(11) =24a(12) =26a(13) =9a(14) =10a(15) =32a(16) =11a(17) =36a(18) =38a(19) =13a(20) =42a(21) =44a(22) =15a(23) =16a(24) =50a(25) =17a(26) =18a(27) =56a(28) =19a(29) =60