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 A380105 #9 Mar 19 2025 05:53:45 %S A380105 1,3,5,12,11,20,24,33,33,52,51,68,70,90,93,117,115,143,147,175,174, %T A380105 210,210,245,248,285,287,330,328,375,378,423,423,477,478,530,532,588, %U A380105 590,652,649,713,717,780,781,852,852,923,925,1000,1001,1080,1078,1160,1165,1245,1245,1335,1335 %N A380105 Perimeter-magic triangles of order 3 with magic sum n, bracelet symmetry, and minimum term 1. %C A380105 The setup is similar to A380853: triangles with 3 positive integers per side, all 6 values distinct, each side's sum equal to n, respecting the bracelet symmetry (rotations and flips generate equivalent triangles). But here 1 must be present at least once among the 6 values. The formula reflects that one can generate magic triangles where 1 is no longer present by adding 1 to each of the 6 integers, which increases the magic sum by 3. %H A380105 R. J. Mathar, <a href="https://zenodo.org/records/15001365">Generating perimeter-magic polygons</a>, C++ (2025) %F A380105 a(n) = A380853(n)-A380853(n-3). %F A380105 G.f.: A380853(x)*(1-x^3). %Y A380105 Cf. A380853 (perimeter-magic triangles). %K A380105 nonn %O A380105 9,2 %A A380105 _R. J. Mathar_, Mar 11 2025