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 A173126 #11 Jun 02 2025 02:41:19
%S A173126 0,0,0,0,1,2,7,14,36,72,165,330,715,1430,3004,6008,12393,24786,50559,
%T A173126 101118,204820,409640,826045,1652090,3321891,6643782,13333932,
%U A173126 26667864,53457121,106914242,214146295,428292590,857417220,1714834440,3431847189
%N A173126 sum_{k=floor[(n+5)/2] mod 5} C(n,k).
%C A173126 Lesser of number of closed walks of length n from a node on a pentagon and number of walks of length n between two adjacent nodes on a pentagon.
%H A173126 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (2, 3, -6, -1, 2).
%F A173126 a(n) =A173125(n)-A000045(n+1). a(2n) =A052964(2n-1); a(2n+1) =A054877(2n+1) =2*a(2n).
%F A173126 a(n)= 2*a(n-1) +3*a(n-2) -6*a(n-3) -a(n-4) +2*a(n-5). G.f.: x^4/((1-2*x) * (x^2+x-1) * (x^2-x-1)). [From _R. J. Mathar_, Feb 19 2010]
%e A173126 For n=15, k=10 mod 5 gives k=0, 5, 10, or 15, and C(15,0)+C(15,5)+C(15,10)+C(15,15) = 1+3003+3003+1, so a(15)=6008
%t A173126 LinearRecurrence[{2,3,-6,-1,2},{0,0,0,0,1},40] (* _Harvey P. Dale_, May 05 2018 *)
%K A173126 nonn
%O A173126 0,6
%A A173126 _Henry Bottomley_, Feb 10 2010