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 A109009 #29 Oct 18 2019 10:23:52
%S A109009 5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,
%T A109009 1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,
%U A109009 1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5,1,1,1,1,5
%N A109009 a(n) = gcd(n,5).
%H A109009 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,0,1).
%F A109009 a(n) = 1 + 4*[5|n], where [x|y] = 1 when x divides y, 0 otherwise.
%F A109009 a(n) = a(n-5).
%F A109009 Multiplicative with a(p^e, 5) = gcd(p^e, 5). - _David W. Wilson_, Jun 12 2005
%F A109009 From _R. J. Mathar_, Apr 04 2011: (Start)
%F A109009 Dirichlet g.f.: zeta(s)*(1+4/5^s).
%F A109009 G.f.: ( -5-x-x^2-x^3-x^4 ) / ( (x-1)*(1+x+x^2+x^3+x^4) ). (End)
%F A109009 a(n) = 4*floor(1/2*cos((2*n*Pi)/5)+1/2) + 1.
%F A109009 = 4*floor(((n-1) mod 5)/4) + 1. - _Gary Detlefs_, Dec 28 2011
%t A109009 GCD[Range[0,100],5] (* or *) PadRight[{},120,{5,1,1,1,1}] (* _Harvey P. Dale_, Jun 29 2018 *)
%o A109009 (PARI) a(n)=gcd(n,5) \\ _Charles R Greathouse IV_, Oct 07 2015
%Y A109009 Cf. A109004.
%K A109009 nonn,easy,mult
%O A109009 0,1
%A A109009 _Mitch Harris_