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 A151980 #19 May 05 2024 10:41:30
%S A151980 0,1,5,16,20,21,25,36,40,41,45,56,60,61,65,76,80,81,85,96,100,101,105,
%T A151980 116,120,121,125,136,140,141,145,156,160,161,165,176,180,181,185,196,
%U A151980 200,201,205,216,220,221,225,236,240,241,245,256,260,261,265,276,280,281,285,296
%N A151980 Numbers n such that n^2 - n is divisible by 20.
%H A151980 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,1,-1).
%F A151980 G.f.: x^2*(1+4*x+11*x^2+4*x^3)/((1-x)^2*(1+x)*(1+x^2)). [_Colin Barker_, Jan 14 2012]
%F A151980 a(n) = a(n-1)+a(n-4)-a(n-5). [_Colin Barker_, Nov 29 2012]
%t A151980 Select[Range[0,300],Divisible[#^2-#,20]&] (* or *) LinearRecurrence[ {1,0,0,1,-1},{0,1,5,16,20},60] (* _Harvey P. Dale_, Aug 16 2020 *)
%K A151980 nonn,easy
%O A151980 1,3
%A A151980 _N. J. A. Sloane_, Aug 23 2009