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%I #18 Feb 19 2024 10:37:13
%S 4,20,100,500,5000,50000,500000,5000000,50000000,500000000,5000000000,
%T 50000000000,500000000000,5000000000000,50000000000000,
%U 500000000000000,5000000000000000,50000000000000000,500000000000000000
%N Period of powers of 3 mod 10^n.
%F 4*5^(n-1), n <= 4.
%F 5*10^(n-2), n >= 5.
%t Table[If[n<5,(4*5^n)/5,10^n/20],{n,20}] (* or *) Join[{4,20,100},NestList[ 10#&,500,20]] (* _Harvey P. Dale_, May 31 2017 *)
%o (PARI) a(n)=if(n<5,4*5^n/5,10^n/20) \\ _Charles R Greathouse IV_, Mar 26 2016
%Y Cf. A000244, A001148, A001218.
%K nonn,base,easy
%O 1,1
%A _V. Raman_, Sep 01 2012