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 A204118 #9 Aug 02 2019 04:13:13
%S A204118 2,1,1,1,3,1,1,1,1,1,1,1,5,1,1,1,1,1,1,1,1,1,1,1,7,1,1,1,1,1,1,1,1,1,
%T A204118 1,1,1,1,1,1,11,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,13,1,1,1,1,1,1,
%U A204118 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,17,1,1,1,1,1,1,1,1,1,1,1,1,1
%N A204118 Symmetric matrix based on f(i,j) = gcd(prime(i), prime(j)), by antidiagonals.
%C A204118 A204118 represents the matrix M given by f(i,j) = gcd(prime(i), prime(j)) for i >= 1 and j >= 1. See A204119 for characteristic polynomials of principal submatrices of M, with interlacing zeros. See A204016 for a guide to other choices of M.
%e A204118 Northwest corner:
%e A204118 2 1 1 1 1
%e A204118 1 3 1 1 1
%e A204118 1 1 5 1 1
%e A204118 1 1 1 7 1
%e A204118 1 1 1 1 11
%t A204118 f[i_, j_] := GCD[Prime[i], Prime[j]];
%t A204118 m[n_] := Table[f[i, j], {i, 1, n}, {j, 1, n}]
%t A204118 TableForm[m[8]] (* 8 X 8 principal submatrix *)
%t A204118 Flatten[Table[f[i, n + 1 - i],
%t A204118 {n, 1, 15}, {i, 1, n}]] (* A204118 *)
%t A204118 p[n_] := CharacteristicPolynomial[m[n], x];
%t A204118 c[n_] := CoefficientList[p[n], x]
%t A204118 TableForm[Flatten[Table[p[n], {n, 1, 10}]]]
%t A204118 Table[c[n], {n, 1, 12}]
%t A204118 Flatten[%] (* A204119 *)
%t A204118 TableForm[Table[c[n], {n, 1, 10}]]
%Y A204118 Cf. A204119, A204016, A202453.
%K A204118 nonn,tabl
%O A204118 1,1
%A A204118 _Clark Kimberling_, Jan 11 2012