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 A231655 #18 Mar 23 2020 19:16:13 %S A231655 1,1,1,1,1,1,1,2,4,6,4,2,1,1,3,10,25,41,48,41,25,10,3,1,1,4,22,87,244, %T A231655 526,870,1110,1110,870,526,244,87,22,4,1,1,5,41,238,1029,3450,9147, %U A231655 19524,34104,49231,59038,59038,49231,34104,19524,9147,3450,1029,238 %N A231655 Triangle T(n, k) read by rows giving number of non-equivalent ways to choose k points in an equilateral triangle grid of side n. %C A231655 Number of orbits under dihedral group D_6 of order 6. - _N. J. A. Sloane_, Sep 12 2019 %H A231655 Heinrich Ludwig, <a href="/A231655/b231655.txt">Table of n, a(n) for n = 0..173</a> %e A231655 Triangle T(n, k) is irregularly shaped: 0 <= k <= n*(n+1)/2+1. The first row corresponds to n = 1, the first column corresponds to k = 0. Rows are palindromic. %e A231655 1, 1; %e A231655 1, 1, 1, 1; %e A231655 1, 2, 4, 6, 4, 2, 1; %e A231655 1, 3, 10, 25, 41, 48, 41, 25, 10, 3, 1; %e A231655 ... %e A231655 There are T(3, 2) = 4 nonisomorphic choices of 2 points (X) in an equilateral triangle grid of side 3: %e A231655 X . . X %e A231655 . . X X . . X . %e A231655 . X . . . . X . X . . . %Y A231655 Columns k=1..5 are A001399, A227327, A230723, A231653, A231654. %Y A231655 Cf. A054252, A283113, A289709, A326611. %K A231655 nonn,tabf %O A231655 0,8 %A A231655 _Heinrich Ludwig_, Nov 14 2013