a(3n)=10^n. a(3n+1)=4*10^n. a(3n+2)=7*10^n.

Terms

a(0) =1a(1) =4a(2) =7a(3) =10a(4) =40a(5) =70a(6) =100a(7) =400a(8) =700a(9) =1000a(10) =4000a(11) =7000a(12) =10000a(13) =40000a(14) =70000a(15) =100000a(16) =400000a(17) =700000a(18) =1000000a(19) =4000000a(20) =7000000a(21) =10000000a(22) =40000000a(23) =70000000a(24) =100000000a(25) =400000000a(26) =700000000a(27) =1000000000a(28) =4000000000a(29) =7000000000