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 A380913 #11 Feb 15 2025 23:06:22
%S A380913 10,46,85,166,235,274,514,694,901,1135,1219,1306,1585,1891,2461,2839,
%T A380913 3106,3385,3826,3979,4135,5311,5674,6049,6835,7246,8551,9481,10966,
%U A380913 11485,11749,12286,12559,13969,15151,15454,17335,18649,18985,19666,21421,21781,22879,23626
%N A380913 Squarefree semiprimes that are centered triangular numbers.
%C A380913 4=2^2, 361=19^2, 6241=79^2 are centered triangular numbers but not squarefree.
%e A380913 10 = 2 * 5 which is the 3rd centered triangular number.
%e A380913 46 = 2 * 23 which is the 6th centered triangular number.
%e A380913 85 = 5 * 17 which is the 7th centered triangular number.
%t A380913 Select[Table[3*n*(n-1)/2 + 1, {n, 1, 130}], FactorInteger[#][[;; , 2]] == {1, 1} &] (* _Amiram Eldar_, Feb 08 2025 *)
%Y A380913 Intersection of A005448 and A006881.
%Y A380913 Cf. A184481, A359624, A359845.
%K A380913 nonn
%O A380913 1,1
%A A380913 _Massimo Kofler_, Feb 08 2025