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 A187491 #10 Feb 16 2025 08:33:14
%S A187491 1,2,3,6,10,21,32,49,65,101,150,224,305,413,525,707,908,1174,1479,
%T A187491 1871,2269,2826,3396,4138,4967,5991,6917,8244,9673,11328,12958,15091,
%U A187491 17112,19771,22468,25485,28870,32861,36298,40969,45615,51015
%N A187491 Number of 4-element nondividing subsets of {1, 2, ..., n}.
%C A187491 A set is called nondividing if no element divides the sum of any nonempty subset of the other elements.
%H A187491 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/NondividingSet.html">Nondividing Set</a>
%e A187491 a(10) = 1 because there is one 4-element nondividing subset of {1,2,...,10}: {6,7,9,10}.
%Y A187491 Column 4 of triangle A187489. Cf. A068063.
%K A187491 nonn
%O A187491 10,2
%A A187491 _Alois P. Heinz_, Mar 10 2011