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 A225875 #30 Feb 03 2019 13:44:22
%S A225875 1,2,2,3,3,4,4,5,6,6,7,7,8,8,9,10,10,11,11,12,12,13,14,14,15,15,16,16,
%T A225875 17,18,18,19,19,20,20,21,22,22,23,23,24,24,25,26,26,27,27,28,28,29,30,
%U A225875 30,31,31,32,32,33,34,34,35,35,36,36,37,38,38,39,39,40,40,41,42,42,43
%N A225875 We write the 1 + 4*k numbers once and twice the others.
%C A225875 First differences are periodic with period 7.
%H A225875 <a href="/index/Rec#order_08">Index entries for linear recurrences with constant coefficients</a>, signature (1, 0, 0, 0, 0, 0, 1, -1).
%F A225875 a(n+1) = 1 + 4*floor(n/7) + [0,1,1,2,2,3,3].
%F A225875 G.f.: x*(1 + x + x^3 + x^5)/((1-x)^2 * (1 + x + x^2 + x^3 + x^4 + x^5 + x^6)).
%F A225875 a(n) = n - floor(3*n/7). - _Wesley Ivan Hurt_, Sep 29 2017
%t A225875 t = {}; Do[If[Mod[n, 4] == 1, AppendTo[t, n], AppendTo[t, {n, n}]], {n, 50}]; Flatten[t] (* _T. D. Noe_, May 23 2013 *)
%t A225875 LinearRecurrence[{1, 0, 0, 0, 0, 0, 1, -1},{1, 2, 2, 3, 3, 4, 4, 5},74] (* _Ray Chandler_, Aug 26 2015 *)
%t A225875 Table[If[Mod[n, 4] == 1, n, {n, n}], {n, 50}] // Flatten (* or *) Drop[ Flatten[ Table[{n,n},{n,50}]],{1,-1,8}] (* _Harvey P. Dale_, Feb 03 2019 *)
%Y A225875 Cf. A132270.
%K A225875 nonn,easy
%O A225875 1,2
%A A225875 _José María Grau Ribas_, May 19 2013
%E A225875 Corrected, extended, and edited by _Ralf Stephan_, May 20 2013