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 A257845 #16 Aug 15 2021 17:43:15
%S A257845 0,0,0,0,0,0,1,2,3,4,0,2,4,6,8,0,3,6,9,12,0,4,8,12,16,0,5,10,15,20,0,
%T A257845 6,12,18,24,0,7,14,21,28,0,8,16,24,32,0,9,18,27,36,0,10,20,30,40,0,11,
%U A257845 22,33,44,0,12,24,36,48,0,13,26,39,52,0,14,28,42,56
%N A257845 a(n) = floor(n/5) * (n mod 5).
%C A257845 Equivalently, write n in base 5, multiply the last digit by the number with its last digit removed.
%H A257845 Colin Barker, <a href="/A257845/b257845.txt">Table of n, a(n) for n = 0..1000</a>
%H A257845 <a href="/index/Rec#order_10">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,0,2,0,0,0,0,-1).
%F A257845 a(n) = 2*a(n-5)-a(n-10). - _Colin Barker_, May 11 2015
%F A257845 G.f.: x^6*(4*x^3+3*x^2+2*x+1) / ((x-1)^2*(x^4+x^3+x^2+x+1)^2). - _Colin Barker_, May 11 2015
%t A257845 LinearRecurrence[{0,0,0,0,2,0,0,0,0,-1},{0,0,0,0,0,0,1,2,3,4},80] (* _Harvey P. Dale_, Aug 15 2021 *)
%o A257845 (PARI) a(n,b=5)=(n=divrem(n,b))[1]*n[2]
%o A257845 (PARI) concat([0,0,0,0,0,0], Vec(x^6*(4*x^3+3*x^2+2*x+1) / ((x-1)^2*(x^4+x^3+x^2+x+1)^2) + O(x^100))) \\ _Colin Barker_, May 11 2015
%Y A257845 Cf. A142150 (the base 2 analog), A115273, A257844 - A257850.
%K A257845 nonn,easy
%O A257845 0,8
%A A257845 _M. F. Hasler_, May 10 2015