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 A274777 #42 Apr 18 2019 04:43:03 %S A274777 1,0,2,1,0,0,4,4,2,1,3,0,0,0 %N A274777 Irregular triangle read by rows: T(n,k) = number of arrangements of n circles in the affine plane forming k regions, excluding the regions that do not belong to the circles. %C A274777 In other words: not counting the regions between circles. %C A274777 Consider the arrangements of n circles described in A250001. %C A274777 Note that the sum of the 4th row must be equal to A250001(4) = 173. %F A274777 T(n,k) = A274819(n,k)/k. %e A274777 Triangle begins: %e A274777 1; %e A274777 0, 2, 1; %e A274777 0, 0, 4, 4, 2, 1, 3; %e A274777 0, 0, 0, ... %e A274777 ... %e A274777 For n = 3 and k = 5 there are 2 arrangements of 3 circles in the affine plane forming 5 regions, excluding the regions that do not belong to the circles, so T(3,5) = 2. %e A274777 For n = 3 and k = 6 there is only one arrangement of 3 circles in the affine plane forming 6 regions, excluding the regions that do not belong to the circles, so T(3,6) = 1. %e A274777 Of course, there is a right triangle of all zeros starting from the second row. %Y A274777 Sum of n-th row = A250001(n). %Y A274777 First differs from A274776 at a(10). %Y A274777 Cf. A250001, A249752, A252158, A261070, A274819, A274823. %K A274777 nonn,tabf,hard,more %O A274777 1,3 %A A274777 _Omar E. Pol_, Jul 06 2016