a(n) = 2^((n-1) mod 2)*5*10^floor((n-1)/2).

Terms

a(0) =1a(1) =5a(2) =10a(3) =50a(4) =100a(5) =500a(6) =1000a(7) =5000a(8) =10000a(9) =50000a(10) =100000a(11) =500000a(12) =1000000a(13) =5000000a(14) =10000000a(15) =50000000a(16) =100000000a(17) =500000000a(18) =1000000000a(19) =5000000000a(20) =10000000000a(21) =50000000000a(22) =100000000000a(23) =500000000000a(24) =1000000000000