Write n in binary and replace 0 with 0000.

Terms

a(0) =1a(1) =10000a(2) =11a(3) =100000000a(4) =100001a(5) =110000a(6) =111a(7) =1000000000000a(8) =1000000001a(9) =1000010000a(10) =1000011a(11) =1100000000a(12) =1100001a(13) =1110000a(14) =1111a(18) =10000000011