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 A366755 #9 Feb 12 2024 15:17:47
%S A366755 1,1,1,3,8,48,387,6240,178176
%N A366755 Number of 1-tough unlabeled graphs on n vertices.
%H A366755 Wikipedia, <a href="https://en.wikipedia.org/wiki/Graph_toughness">Graph toughness</a>.
%F A366755 a(n) <= A002218(n) for n >= 2 because all 1-tough graphs (except the 1-node graph) are 2-connected.
%e A366755 For n = 5, all but two of the A002218(5) = 10 2-connected graphs are 1-tough, so a(5) = 8. The exceptions are the complete bipartite graph K_{2,3} and the complete tripartite graph K_{1,1,3}. To see that these graphs are not 1-tough, note that, in both cases, two vertices can be removed resulting in a graph with three components (isolated vertices).
%Y A366755 Cf. A002218, A007031, A366315, A366756.
%K A366755 nonn,more
%O A366755 1,4
%A A366755 _Pontus von Brömssen_, Oct 20 2023