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 A335490 #28 Sep 13 2020 15:10:47 %S A335490 1,2,3,3,1,2,4,2,3,5,5,6,1,4,7,6,4,5,7,8,9,7,5,6,1,4,10,8,8,9,4,2,3,5, %T A335490 6,10,9,7,8,3,1,2,10,5,4,10,8,9,6,2,3,7,11,12,13,11,12,7,10,5,1,9,8,6, %U A335490 14,15,12,10,11,13,6,4,14,7,9,8,16,17,13,11 %N A335490 Isosceles triangle read by rows in which each term is the least positive integer satisfying the condition that no row, diagonal, or antidiagonal contains a repeated term. %C A335490 The n-th instance of 1 occurs at index A001844(n-1). %C A335490 Records occur at 1, 2, 3, 7, 10, 12, 15, 20, 21, 27, 53, 54, 55, 65, ... %F A335490 a(n) = A296339(n-1) + 1. - _Rémy Sigrist_, Sep 13 2020 %e A335490 Triangle begins: %e A335490 1 %e A335490 2 3 %e A335490 3 1 2 %e A335490 4 2 3 5 %e A335490 5 6 1 4 7 %e A335490 6 4 X ... %e A335490 The value for X is 5 because 1, 2, and 3 are on the diagonal; 4 and 6 are on the antidiagonal; and 4 and 6 are in the row. Therefore 5 is the smallest value that can be inserted so that no diagonal, antidiagonal, or row contains a repeated term. %Y A335490 Analogs for other tilings: A269526 (square), A334049 (triangular). %Y A335490 Cf. A001844, A274650, A274651, A288530, A288531, A296339. %K A335490 tabl,nonn,more %O A335490 1,2 %A A335490 _Alec Jones_ and _Peter Kagey_, Sep 12 2020