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 A177990 #13 Jul 22 2025 08:10:26
%S A177990 1,0,1,0,1,1,0,1,0,1,0,1,0,1,1,0,1,0,1,0,1,0,1,0,1,0,1,1,0,1,0,1,0,1,
%T A177990 0,1,0,1,0,1,0,1,0,1,1,0,1,0,1,0,1,0,1,0,1
%N A177990 Triangle read by rows, variant of A070909, a cellular automaton.
%C A177990 A070909 * A177990 = A177991
%C A177990 A007318 * A177990 = A177992
%C A177990 A177990 * A007318 = A177993
%F A177990 Infinite lower triangular matrix, variant of A070909: Columns alternate between (1,0,0,0,...) and (1,1,1,...); whereas A070909 leftmost column = (1,1,1,...).
%e A177990 First few rows of the triangle =
%e A177990 1;
%e A177990 0, 1;
%e A177990 0, 1, 1;
%e A177990 0, 1, 0, 1;
%e A177990 0, 1, 0, 1, 1;
%e A177990 0, 1, 0, 1, 0, 1;
%e A177990 0, 1, 0, 1, 0, 1, 1;
%e A177990 0, 1, 0, 1, 0, 1, 0, 1;
%e A177990 0, 1, 0, 1, 0, 1, 0, 1, 1;
%e A177990 0, 1, 0, 1, 0, 1, 0, 1, 0, 1;
%e A177990 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1;
%e A177990 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1;
%e A177990 ...
%t A177990 rows = 10; ca = CellularAutomaton[28, {{1}, 0}, rows]; Table[ca[[k, 2 ;; k]], {k, 2, rows+1}] // Flatten (* _Jean-François Alcover_, Jul 12 2017 *)
%Y A177990 Cf. A070909, A177991, A177992, A177993.
%K A177990 nonn,tabl
%O A177990 1,1
%A A177990 _Gary W. Adamson_, May 16 2010