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%I #17 Jul 01 2021 08:22:59
%S 1,1,1,2,2,1,1,2,3,2,1,2,3,3,1,1,2,3,2,1,1,2,3,4,4,3,2,3,4,4,1,1,2,3,
%T 2,1,1,2,3,4,5,4,3,2,1,2,3,4,5,5,4,3,4,5,5,1,1,2,3,2,1,1,2,3,4,5,4,3,
%U 2,1,1,2,3,4,5,6,6,5,4,3,2,3,4,5,6,6,5,4,5,6,6
%N Irregular triangle read by rows: the n-th row gives the row indices of the matrix of 1..n^2 filled successively back and forth along antidiagonals.
%H Stefano Spezia, <a href="/A343853/b343853.txt">First 30 rows of the triangle, flattened</a>
%e The triangle begins:
%e 1
%e 1 1 2 2
%e 1 1 2 3 2 1 2 3 3
%e 1 1 2 3 2 1 1 2 3 4 4 3 2 3 4 4
%e ...
%t a={};For[n=1,n<=6,n++,For[d=1,d<=n,d++, If[OddQ[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[OddQ[d],i= n; For[k=1,k<=2n-d,k++,AppendTo[a,i-k+1]],If[EvenQ[d],i=d-n+1;For[k=1,k<=2n-d,k++, AppendTo[a,i+k-1]]]]]]; a
%Y Cf. A000290 (row length), A002411 (row sums), A060747 (number of antidiagonals), A078475, A319573, A343854 (column indices).
%K nonn,look,tabf
%O 1,4
%A _Stefano Spezia_, May 01 2021