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 A337790 #10 Sep 26 2020 02:58:31 %S A337790 3,57,306,837,1335,2073,3033,4395,6147,8469,11253,14907,19263,24819, %T A337790 31197,38823,47619 %N A337790 Number of vertices in a Y-shaped polygon with equal arms of length n (see the Comments in A335861 for definition). %C A337790 See A335861 for a definition of the polygon and images for the number of regions. %H A337790 Scott R. Shannon, <a href="/A337790/a337790.png">Image for the figure showing the vertex counts for n=1</a>. %H A337790 Scott R. Shannon, <a href="/A337790/a337790_1.png">Image for the figure showing the vertex counts for n=2</a>. %H A337790 Scott R. Shannon, <a href="/A337790/a337790_2.png">Image for the figure showing the vertex counts for n=3</a>. %H A337790 Scott R. Shannon, <a href="/A337790/a337790_3.png">Image for the figure showing the vertex counts for n=4</a>. %H A337790 Scott R. Shannon, <a href="/A337790/a337790_4.png">Image for the figure showing the vertex counts for n=5</a>. %H A337790 Scott R. Shannon, <a href="/A337790/a337790_5.png">Image for the figure showing the vertex counts for n=6</a>. %e A337790 a(0) = 3. A single triangle with no other polygons has three vertices. %e A337790 a(1) = 57. With one square adjacent to each of the triangles sides the resulting line segments form 51 vertices shared by four polygons, 3 vertices shared by six polygons, and 3 vertices shared by seven polygons. This gives a total of 57 vertices. See the first linked image. %Y A337790 Cf. A335861 (number of regions), A331456, A331452, A306302, A092867, A007678. %K A337790 nonn,more %O A337790 0,1 %A A337790 _Scott R. Shannon_ and _N. J. A. Sloane_, Sep 22 2020