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 A119663 #12 Sep 17 2024 12:34:23 %S A119663 1,3,6,10,15,21,28,36,45,55,91,136,153,171,253,325,351,496,703,1081, %T A119663 1225,1431,1711,1891,2701,3321,3403,4753,5671,7381,8128,12403,13203, %U A119663 13861,15931,18721,25651,29161,29403,31375,32896,34453,38503,49141 %N A119663 Triangular numbers with at most two distinct prime factors. %C A119663 1 and 3 are the only terms with less than two prime factors. %H A119663 Klaus Brockhaus, <a href="/A119663/b119663.txt">Table of n, a(n) for n=1..1000</a> %e A119663 a(6) = 3 * 7, a(7) = 2^2 * 7, a(8) = 2^2 * 3^2. %t A119663 Select[Accumulate[Range[0, 320]] ,PrimeNu[#]<3&] (* _James C. McMahon_, Sep 17 2024 *) %o A119663 (PARI) for(n=1, 320, k=binomial(n+1, 2); if(omega(k)<=2, print1(k, ", "))) \\ _Klaus Brockhaus_, Jul 30 2006 %Y A119663 Cf. A000217, A068443, A005384. %K A119663 easy,nonn %O A119663 1,2 %A A119663 _Greg Huber_, Jul 28 2006 %E A119663 More terms from _Klaus Brockhaus_, Jul 30 2006 and May 21 2008