Period of powers of 7 mod 10^n.

Terms

a(0) =4a(1) =4a(2) =20a(3) =100a(4) =500a(5) =5000a(6) =50000a(7) =500000a(8) =5000000a(9) =50000000a(10) =500000000a(11) =5000000000a(12) =50000000000a(13) =500000000000