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 A204030 #8 Aug 02 2019 04:12:12
%S A204030 2,1,1,2,3,2,1,1,1,1,2,1,4,1,2,1,3,1,1,3,1,2,1,2,5,2,1,2,1,1,1,1,1,1,
%T A204030 1,1,2,3,4,1,6,1,4,3,2,1,1,1,1,1,1,1,1,1,1,2,1,2,1,2,7,2,1,2,1,2,1,3,
%U A204030 1,5,3,1,1,3,5,1,3,1,2,1,4,1,2,1,8,1,2,1,4,1,2,1,1,1,1,1,1,1,1
%N A204030 Symmetric matrix based on f(i,j) = gcd(i+1, j+1), by antidiagonals.
%C A204030 A204030 represents the matrix M given by f(i,j) = gcd(i+1, j+1) for i >= 1 and j >= 1. See A204031 for characteristic polynomials of principal submatrices of M, with interlacing zeros. See A204016 for a guide to other choices of M.
%e A204030 Northwest corner:
%e A204030 2 1 2 1 2 1 2 1
%e A204030 1 3 1 1 3 1 1 3
%e A204030 2 1 4 1 2 1 4 1
%e A204030 1 1 1 5 1 1 1 1
%e A204030 2 3 2 1 6 1 2 3
%e A204030 1 1 1 1 1 7 1 1
%t A204030 f[i_, j_] := GCD[i + 1, j + 1];
%t A204030 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204030 TableForm[m[8]] (* 8 X 8 principal submatrix *)
%t A204030 Flatten[Table[f[i, n + 1 - i],
%t A204030 {n, 1, 15}, {i, 1, n}]] (* A204030 *)
%t A204030 p[n_] := CharacteristicPolynomial[m[n], x];
%t A204030 c[n_] := CoefficientList[p[n], x]
%t A204030 TableForm[Flatten[Table[p[n], {n, 1, 10}]]]
%t A204030 Table[c[n], {n, 1, 12}]
%t A204030 Flatten[%] (* A204111 *)
%t A204030 TableForm[Table[c[n], {n, 1, 10}]]
%Y A204030 Cf. A204111, A204016, A202453.
%K A204030 nonn,tabl
%O A204030 1,1
%A A204030 _Clark Kimberling_, Jan 11 2012