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 A187331 #19 Nov 22 2017 15:13:04
%S A187331 0,1,4,6,10,11,14,16,20,21,24,26,30,31,34,36,40,41,44,46,50,51,54,56,
%T A187331 60,61,64,66,70,71,74,76,80,81,84,86,90,91,94,96,100,101,104,106,110,
%U A187331 111,114,116,120,121,124,126,130,131,134,136,140,141,144,146,150,151,154,156,160,161,164
%N A187331 a(n) = Sum_{k=1..4} floor(k*n/4).
%H A187331 M. F. Hasler, <a href="/A187331/b187331.txt">Table of n, a(n) for n = 0..1000</a>
%F A187331 From _Chai Wah Wu_, Jun 07 2016: (Start)
%F A187331 a(n) = a(n-1) + a(n-4) - a(n-5) for n > 4.
%F A187331 G.f.: x*(4*x^3 + 2*x^2 + 3*x + 1)/(x^5 - x^4 - x + 1). (End)
%F A187331 a(n) = n + A187326(n), where A187326(n) = [n/4] + [n/2] + [3n/4], [.] = floor. - _M. F. Hasler_, Nov 21 2017
%t A187331 t=Table[Sum[Floor[k*n/4], {k,1,4}],{n,0,200}] (* A187331 *)
%t A187331 Complement[Range[Length[t]], t] (* A187332 *)
%o A187331 (PARI) A187331(n)=n\4+n\2+3*n\4+n \\ _M. F. Hasler_, Nov 21 2017
%Y A187331 Cf. A187332 (complement), A187326 (= a(n) - n), A187333.
%K A187331 nonn
%O A187331 0,3
%A A187331 _Clark Kimberling_, Mar 08 2011