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 A187490 #12 Feb 16 2025 08:33:14
%S A187490 1,2,6,12,22,31,49,70,99,128,176,216,284,343,423,515,633,722,860,1007,
%T A187490 1173,1333,1552,1729,1989,2223,2502,2809,3138,3416,3819,4226,4658,
%U A187490 5049,5570,6016,6601,7146,7719,8371,9100,9686,10461,11208,12039
%N A187490 Number of 3-element nondividing subsets of {1, 2, ..., n}.
%C A187490 A set is called nondividing if no element divides the sum of any nonempty subset of the other elements.
%H A187490 Alois P. Heinz, <a href="/A187490/b187490.txt">Table of n, a(n) for n = 7..200</a>
%H A187490 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/NondividingSet.html">Nondividing Set.</a>
%e A187490 a(7) = 1 because there is one 3-element nondividing subset of {1,2,3,4,5,6,7}: {4,6,7}.
%e A187490 a(9) = 6: {4,6,7}, {4,6,9}, {5,6,8}, {5,8,9}, {6,7,9}, {6,8,9}.
%Y A187490 Column 3 of triangle A187489. Cf. A068063.
%K A187490 nonn
%O A187490 7,2
%A A187490 _Alois P. Heinz_, Mar 10 2011