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 A331782 #15 Feb 14 2020 19:35:36 %S A331782 3,7,21,25,63,67,129,133,219,223,333,337,471,475,621,637,819,823,1029, %T A331782 1021,1263,1267,1521,1477,1803,1807,2109,2113,2439,2431,2793,2797, %U A331782 3171,3175,3549,3577,3999,4003,4449,4417,4923,4903,5421,5425,5859,5947,6489,6397,7059,7063,7653,7657,8271,8275,8889 %N A331782 Total number of vertices in graph formed by the straight line segments connecting the edges of an equilateral triangle with the n-1 points resulting from a subdivision of the sides into n equal pieces, counting coinciding intersection points only once. %C A331782 a(n) <= 3*(n^2-n+1), with equality iff n is odd and not a member of A332378. A331423 gives the difference between a(n) and the upper bound. %H A331782 N. J. A. Sloane, <a href="/A331782/b331782.txt">Table of n, a(n) for n = 1..1000</a> %F A331782 a(n) = A091908(n) + 3*n. %Y A331782 Cf. A091908, A092098 (number of cells), A332376 (number of edges), A332378, A331423. %K A331782 nonn %O A331782 1,1 %A A331782 _N. J. A. Sloane_, Feb 13 2020