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 A061726 #15 Jul 01 2018 17:14:22
%S A061726 4,12,30,50,60,84,140,180,180,220,330,390,364,420,600,680,612,684,950,
%T A061726 1050,924,1012,1380,1500,1300,1404,1890,2030,1740,1860,2480,2640,2244,
%U A061726 2380,3150,3330,2812,2964,3900,4100,3444,3612,4730,4950,4140,4324,5640
%N A061726 If n-th triangular number (A000217(n)) is odd, multiply it by 4; if even, multiply it by 5.
%D A061726 Norman Sullivan, Test Your Own IQ, Workman Publishing Co. New York, NY, 1994, pp. 48, 50.
%H A061726 Harry J. Smith, <a href="/A061726/b061726.txt">Table of n, a(n) for n = 1..1000</a>
%F A061726 a(n) = t*(5 - (t mod 2)) where t is the n-th triangular number (A000217(n)). - _Jon E. Schoenfield_, Jul 01 2018
%F A061726 G.f.: -2*(2*x^6+9*x^4-4*x^3+9*x^2+2)*x/((x-1)^3*(x^2+1)^3). - _Alois P. Heinz_, Jul 01 2018
%o A061726 (PARI) { for (n=1, 1000, a=n*(n + 1)/2; if (a%2, a*=4, a*=5); write("b061726.txt", n, " ", a) ) } \\ _Harry J. Smith_, Jul 27 2009
%Y A061726 Cf. A000217, A061727.
%K A061726 easy,nonn
%O A061726 1,1
%A A061726 _Jason Earls_, May 05 2001
%E A061726 More terms from Larry Reeves (larryr(AT)acm.org), May 07 2001