a(n) = A052217(n)/3.

Terms

a(0) =1a(1) =4a(2) =7a(3) =10a(4) =34a(5) =37a(6) =40a(7) =67a(8) =70a(9) =100a(10) =334a(11) =337a(12) =340a(13) =367a(14) =370a(15) =400a(16) =667a(17) =670a(18) =700a(19) =1000a(20) =3334a(21) =3337a(22) =3340a(23) =3367a(24) =3370a(25) =3400a(26) =3667a(27) =3670a(28) =3700a(29) =4000