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 A061727 #14 Jul 01 2018 18:52:31
%S A061727 1,4,3,12,6,30,10,50,15,60,21,84,28,140,36,180,45,180,55,220,66,330,
%T A061727 78,390,91,364,105,420,120,600,136,680,153,612,171,684,190,950,210,
%U A061727 1050,231,924,253,1012,276,1380,300,1500,325,1300,351,1404,378,1890,406
%N A061727 A000217 interleaved with A061726.
%D A061727 Norman Sullivan, Test Your Own IQ, Workman Publishing Co. New York, NY, 1994, pp. 48, 50.
%H A061727 Harry J. Smith, <a href="/A061727/b061727.txt">Table of n, a(n) for n = 1..2000</a>
%t A061727 Array[{#, (4 + Boole[EvenQ@ #]) #} &@ PolygonalNumber@ # &, 28] // Flatten (* _Michael De Vlieger_, Jul 01 2018 *)
%o A061727 (PARI) { f="b061727.txt"; for (n=1, 1000, a=n*(n + 1)/2; if (a%2, t=a*4, t=a*5); write(f, 2*n - 1, " ", a); write(f, 2*n, " ", t) ) } \\ _Harry J. Smith_, Jul 27 2009
%Y A061727 Cf. A000217, A061726.
%K A061727 nonn,dumb,easy
%O A061727 1,2
%A A061727 _Jason Earls_, May 05 2001
%E A061727 More terms from _David Wasserman_, Jun 25 2002