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 A061900 #10 Oct 27 2023 22:00:46
%S A061900 28,36,45,120,136,153,171,276,300,325,351,378,496,528,630,666,780,820,
%T A061900 990,1035,1128,1176,1225,1275,1431,1485,1540,1596,1953,2016,2080,2556,
%U A061900 2628,2775,2850,3160,3240,3321,3828,3916,4005,4095,4560,4656,4753,4851
%N A061900 Triangular numbers that are not squarefree.
%H A061900 Harry J. Smith, <a href="/A061900/b061900.txt">Table of n, a(n) for n = 1..1000</a>
%e A061900 36 and 45 are triangular numbers (A000217) and are not squarefree: 36 = (2^2)(3^2), 45 = (3^2)(5).
%o A061900 (PARI) { n=t=0; for (i=1, 1926, t+=i; if (!issquarefree(t), write("b061900.txt", n++, " ", t)) ) } \\ _Harry J. Smith_, Jul 28 2009
%Y A061900 Cf. A000217, A061304.
%K A061900 nonn
%O A061900 1,1
%A A061900 _Asher Auel_, May 20 2001