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 A067197 #25 Mar 02 2021 05:59:54 %S A067197 7,8,9,15,16,17,18,23,24,25,26,27,31,32,35,36,39,40,44,45,47,48,49,50, %T A067197 53,54,55,56,62,63,64,71,72,74,75,79,80,81,87,88,89,90,95,96,97,98,99, %U A067197 100,103,104,107,108,111,112,116,117,119,120,121,124,125,126,127,128 %N A067197 Numbers k such that k*(k+1)/2 is not squarefree. %C A067197 The asymptotic density of this sequence is 1 - (3/2)*A065474 = 0.5160488515... (Granville and Ramaré, 1996). - _Amiram Eldar_, Mar 02 2021 %H A067197 Amiram Eldar, <a href="/A067197/b067197.txt">Table of n, a(n) for n = 1..10000</a> %H A067197 Andrew Granville and Olivier Ramaré, <a href="https://doi.org/10.1112/S0025579300011608">Explicit bounds on exponential sums and the scarcity of squarefree binomial coefficients</a>, Mathematika, Vol. 43, No. 1 (1996), pp. 73-107; <a href="https://dms.umontreal.ca/~andrew/PDF/ramare.pdf">alternative link</a>. %t A067197 Select[Range[200],!SquareFreeQ[#(#+1)/2]&] (* _Vladimir Joseph Stephan Orlovsky_, Mar 30 2011 *) %o A067197 (PARI) isok(k) = !issquarefree(k*(k+1)/2); \\ _Michel Marcus_, Feb 17 2021 %Y A067197 Complement of A215726. %Y A067197 Cf. A000217, A005117, A013929, A065474. %K A067197 nonn %O A067197 1,1 %A A067197 _Benoit Cloitre_, Feb 19 2002