Period of powers of 3 mod 10^n.

Terms

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