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 A278481 #17 Nov 23 2016 11:30:24 %S A278481 2,4,4,4,6,4,4,6,6,4,4,6,6,6,4,4,6,6,6,6,4,4,6,6,6,6,6,4,4,6,6,6,6,6, %T A278481 6,4,4,6,6,6,6,6,6,6,4,4,6,6,6,6,6,6,6,6,4,4,6,6,6,6,6,6,6,6,6,4,4,6, %U A278481 6,6,6,6,6,6,6,6,6,4,4,6,6,6,6,6,6,6,6,6,6,6,4 %N A278481 Number of neighbors of the n-th term in a full isosceles triangle read by rows. %C A278481 Apart from the left border and the right border, the rest of the elements are 6's. %C A278481 For the same idea but for a right triangle see A278480; for a square array see A278545, for a square spiral see A010731; and for a hexagonal spiral see A010722. %e A278481 The sequence written as an isosceles triangle begins: %e A278481 . %e A278481 . 2; %e A278481 . 4, 4; %e A278481 . 4, 6, 4; %e A278481 . 4, 6, 6, 4; %e A278481 . 4, 6, 6, 6, 4; %e A278481 . 4, 6, 6, 6, 6, 4; %e A278481 . 4, 6, 6, 6, 6, 6, 4; %e A278481 . 4, 6, 6, 6, 6, 6, 6, 4; %e A278481 . 4, 6, 6, 6, 6, 6, 6, 6, 4; %e A278481 . 4, 6, 6, 6, 6, 6, 6, 6, 6, 4; %e A278481 ... %Y A278481 Row sums give A016933. %Y A278481 Left border gives A040002, the same as the right border. %Y A278481 Middle column gives the elements > 1 of A134201, also twice A122553. %Y A278481 Cf. A010722, A010731, A275015, A278317, A278480, A278545. %K A278481 nonn,tabl %O A278481 1,1 %A A278481 _Omar E. Pol_, Nov 23 2016