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 A343854 #15 Jul 01 2021 08:23:39
%S A343854 1,1,2,1,2,1,2,1,1,2,3,3,2,3,1,2,1,1,2,3,4,3,2,1,2,3,4,4,3,4,1,2,1,1,
%T A343854 2,3,4,3,2,1,1,2,3,4,5,5,4,3,2,3,4,5,5,4,5,1,2,1,1,2,3,4,3,2,1,1,2,3,
%U A343854 4,5,6,5,4,3,2,1,2,3,4,5,6,6,5,4,3,4,5,6,6,5,6
%N A343854 Irregular triangle read by rows: the n-th row gives the column indices of the matrix of 1..n^2 filled successively back and forth along antidiagonals.
%H A343854 Stefano Spezia, <a href="/A343854/b343854.txt">First 30 rows of the triangle, flattened</a>
%e A343854 The triangle begins:
%e A343854 1
%e A343854 1 2 1 2
%e A343854 1 2 1 1 2 3 3 2 3
%e A343854 1 2 1 1 2 3 4 3 2 1 2 3 4 4 3 4
%e A343854 ...
%t A343854 a={};For[n=1,n<=6,n++,For[d=1,d<=n,d++, If[EvenQ[d],i=d;For [k=1,k<=d,k++, AppendTo[a,i-k+1]],i=1;For[k=1,k<=d,k++, AppendTo[a,i+k-1]]]];For[d=n+1,d<=2n-1,d++, If[EvenQ[d],i= n; For[k=1,k<=2n-d,k++,AppendTo[a,i-k+1]],If[OddQ[d],i=d-n+1;For[k=1,k<=2n-d,k++, AppendTo[a,i+k-1]]]]]]; a
%Y A343854 Cf. A000290 (row length), A002411 (row sums), A060747 (number of antidiagonals), A078475, A319572, A343853 (row indices).
%K A343854 nonn,look,tabf
%O A343854 1,3
%A A343854 _Stefano Spezia_, May 01 2021