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 A131431 #5 Oct 02 2021 17:33:36 %S A131431 1,0,4,0,0,7,0,0,0,10,0,0,0,0,13,0,0,0,0,0,16,0,0,0,0,0,0,19,0,0,0,0, %T A131431 0,0,0,22,0,0,0,0,0,0,0,0,25,0,0,0,0,0,0,0,0,0,28,0,0,0,0,0,0,0,0,0,0, %U A131431 31,0,0,0,0,0,0,0,0,0,0,0,34,0,0,0,0,0,0,0,0,0,0,0,0,37 %N A131431 3n + 1 preceded by n zeros. %F A131431 A016777 as the diagonal of an infinite lower triangular matrix with the rest zeros. %e A131431 First few rows of the triangle are: %e A131431 1; %e A131431 0, 4; %e A131431 0, 0, 7; %e A131431 0, 0, 0, 10; %e A131431 0, 0, 0, 0, 13; %e A131431 ... %p A131431 ListTools[Flatten]([seq([seq(0, k=1..n), 3*n+1], n=0..12)]); # _Georg Fischer_, Oct 02 2021 %Y A131431 Cf. A016777, A131432. %K A131431 nonn,tabl %O A131431 0,3 %A A131431 _Gary W. Adamson_, Jul 10 2007 %E A131431 a(35) corrected and more terms from _Georg Fischer_, Oct 02 2021