This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A216164 #28 Feb 19 2024 10:49:48
%S A216164 4,4,20,100,500,5000,50000,500000,5000000,50000000,500000000,
%T A216164 5000000000,50000000000,500000000000,5000000000000,50000000000000,
%U A216164 500000000000000,5000000000000000,50000000000000000,500000000000000000
%N A216164 Period of powers of 7 mod 10^n.
%C A216164 Essentially the same as A216099.
%H A216164 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (10).
%F A216164 a(n) = 4*5^(n-2), 1 < n < 5.
%F A216164 a(n) = 5*10^(n-3), n >= 5.
%F A216164 a(n) = 2^(n-3)*5^(n-2) for n>4. a(n) = 10*a(n-1) for n>5. G.f.: 4*x*(125*x^4 +25*x^3 +5*x^2 +9*x -1) / (10*x -1). - _Colin Barker_, Sep 22 2013
%t A216164 Join[{4, 4, 20, 100}, NestList[10*# &, 500, 20]] (* _Paolo Xausa_, Feb 19 2024 *)
%Y A216164 Cf. A000420, A216156, A216099
%K A216164 nonn,base,easy
%O A216164 1,1
%A A216164 _V. Raman_, Sep 02 2012