a(n)=(number of digits <=3)-(number of digits >3) in base 7 representation of n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =-1a(4) =-1a(5) =-1a(6) =2a(7) =2a(8) =2a(9) =2a(10) =0a(11) =0a(12) =0a(13) =2a(14) =2a(15) =2a(16) =2a(17) =0a(18) =0a(19) =0a(20) =2a(21) =2a(22) =2a(23) =2a(24) =0a(25) =0a(26) =0a(27) =0a(28) =0a(29) =0