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 A204116 #9 Aug 02 2019 04:12:34
%S A204116 1,1,1,1,3,1,1,1,1,1,1,3,7,3,1,1,1,1,1,1,1,1,3,1,15,1,3,1,1,1,7,1,1,7,
%T A204116 1,1,1,3,1,3,31,3,1,3,1,1,1,1,1,1,1,1,1,1,1,1,3,7,15,1,63,1,15,7,3,1,
%U A204116 1,1,1,1,1,1,1,1,1,1,1,1,1,3,1,3,1,3,127,3,1,3,1,3,1,1,1,7,1
%N A204116 Symmetric matrix based on f(i,j) = gcd(2^i-1, 2^j-1), by antidiagonals.
%C A204116 A204116 represents the matrix M given by f(i,j) = gcd(2^i-1, 2^j-1) for i >= 1 and j >= 1. See A204117 for characteristic polynomials of principal submatrices of M, with interlacing zeros. See A204016 for a guide to other choices of M.
%e A204116 Northwest corner:
%e A204116 1 1 1 1
%e A204116 1 3 1 3
%e A204116 1 1 7 1
%e A204116 1 3 1 15
%t A204116 f[i_, j_] := GCD[2^i - 1, 2^j - 1];
%t A204116 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204116 TableForm[m[8]] (* 8 X 8 principal submatrix *)
%t A204116 Flatten[Table[f[i, n + 1 - i],
%t A204116 {n, 1, 15}, {i, 1, n}]] (* A204116 *)
%t A204116 p[n_] := CharacteristicPolynomial[m[n], x];
%t A204116 c[n_] := CoefficientList[p[n], x]
%t A204116 TableForm[Flatten[Table[p[n], {n, 1, 10}]]]
%t A204116 Table[c[n], {n, 1, 12}]
%t A204116 Flatten[%] (* A204117 *)
%t A204116 TableForm[Table[c[n], {n, 1, 10}]]
%Y A204116 Cf. A204117, A204016, A202453.
%K A204116 nonn,tabl
%O A204116 1,5
%A A204116 _Clark Kimberling_, Jan 11 2012