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 A353991 #15 May 14 2022 19:51:31 %S A353991 4,12,24,26,36,42,48,60,72,78,84,96,106,108,120,132,144 %N A353991 The regular m-gons with all diagonals drawn that contain internal vertices with vertex-surrounding polygons with 4 sides. See A353876. %C A353991 See A353876 for further details. All terms are even as a regular polygon with an odd number of edges with all diagonals drawn has only simple interior vertices, i.e. all vertices are created by the crossing of only two lines. Each vertex that creates a 4-sided vertex-surrounding polygon is connected to two other edge vertices as well as the central vertex of this polygon. Such vertices must therefore be created by the crossing of at least three lines, so cannot be vertices of a regular polygon with an odd number of sides. %H A353991 Scott R. Shannon, <a href="/A353991/a353991.png">Image of the 12-gon</a>. The vertex-surrounding polygons with 4 sides are highlighted in darkened red. %Y A353991 Cf. A353876, A007569, A007678, A135565, A351045. %K A353991 nonn,more %O A353991 1,1 %A A353991 _Scott R. Shannon_, May 13 2022 %E A353991 a(13)-a(17) added by _Scott R. Shannon_, May 14 2022