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 A168100 #16 Jun 29 2023 12:55:17
%S A168100 10,11,12,13,14,15,16,17,18,19,20,21,21,21,21,21,21,21,21,21,21,21,21,
%T A168100 21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,
%U A168100 21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21,21
%N A168100 a(n) = number of natural numbers m such that n - 10 <= m <= n + 10.
%C A168100 Generalization: If a(n,k) = number of natural numbers m such that n - k <= m <= n + k (k >= 1) then a(n,k) = a(n-1,k) + 1 = n + k for 0 <= n <= k, a(n,k) = a(n-1,k) = 2k + 1 for n >= k + 1 (see, e.g., A158799). a(n) = 10 + n for 0 <= n <= 10, a(n) = 21 for n >= 11.
%H A168100 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (1).
%F A168100 G.f.: (10 - 9*x - x^12)/(1 - x)^2. - _G. C. Greubel_, Jul 12 2016
%t A168100 CoefficientList[Series[(10 - 9*x - x^12)/(1 - x)^2, {x, 0, 25}], x] (* _G. C. Greubel_, Jul 12 2016 *)
%t A168100 Table[Count[Range[n-10,n+10],_?Positive],{n,0,80}] (* or *) PadRight[ {10,11,12,13,14,15,16,17,18,19,20},120,{21}] (* _Harvey P. Dale_, Jan 09 2019 *)
%K A168100 nonn
%O A168100 0,1
%A A168100 _Jaroslav Krizek_, Nov 18 2009