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 A278317 #30 Nov 23 2016 11:29:44 %S A278317 0,1,2,2,3,2,2,4,3,2,2,4,4,3,2,2,4,4,4,3,2,2,4,4,4,4,3,2,2,4,4,4,4,4, %T A278317 3,2,2,4,4,4,4,4,4,3,2,2,4,4,4,4,4,4,4,3,2,2,4,4,4,4,4,4,4,4,3,2,2,4, %U A278317 4,4,4,4,4,4,4,4,3,2,2,4,4,4,4,4,4,4,4,4,4,3,2,2,4,4,4,4,4,4,4,4,4,4,4,3,2 %N A278317 Number of neighbors of each new term in a right triangle read by rows. %C A278317 To evaluate T(n,k) consider only the neighbors of T(n,k) that are present in the triangle when T(n,k) should be a new term in the triangle. %C A278317 Apart from the first column and the first two diagonals the rest of the elements are 4's. %C A278317 For the same idea but for an isosceles triangle see A275015; for a square array see A278290, for a square spiral see A278354; and for a hexagonal spiral see A047931. %e A278317 Triangle begins: %e A278317 0; %e A278317 1, 2; %e A278317 2, 3, 2; %e A278317 2, 4, 3, 2; %e A278317 2, 4, 4, 3, 2; %e A278317 2, 4, 4, 4, 3, 2; %e A278317 2, 4, 4, 4, 4, 3, 2; %e A278317 2, 4, 4, 4, 4, 4, 3, 2; %e A278317 2, 4, 4, 4, 4, 4, 4, 3, 2; %e A278317 2, 4, 4, 4, 4, 4, 4, 4, 3, 2; %e A278317 ... %Y A278317 Apart from the initial zero, row sums give A004767. %Y A278317 Column 1 is A130130. %Y A278317 Columns > 1 give the terms greater than 1 of A158411. %Y A278317 Right border gives 0 together with A007395, also twice A057427. %Y A278317 Second right border gives A122553. %Y A278317 Cf. A047931, A274912, A274913, A275015, A278290, A278317, A278354, %K A278317 nonn,tabl %O A278317 1,3 %A A278317 _Omar E. Pol_, Nov 18 2016