n is equal to the number of 4s in all numbers <= n written in base 6.

Terms

a(0) =42768a(1) =44921a(2) =44922a(3) =44923a(4) =44924a(5) =44925a(6) =46656a(7) =89424a(8) =91577a(9) =91578a(10) =91579a(11) =91580a(12) =91581a(13) =93312a(14) =136080a(15) =138233a(16) =138234a(17) =138235a(18) =138236a(19) =138237a(20) =139968a(21) =182736a(22) =184889a(23) =184890a(24) =184891a(25) =184892a(26) =184893