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 A140875 #14 Jun 08 2023 08:50:06
%S A140875 -2,-4,0,-4,-4,0,-6,-2,-2,2,-6,-2,-6,-6,2,-8,-2,-2,-2,-2,4,-8,-4,0,-8,
%T A140875 -4,-4,4,-10,0,-6,-4,0,-6,-4,6,-10,-4,-2,0,-10,-8,-2,-4,6,-12,-2,-2,
%U A140875 -2,-2,-2,-2,-2,-2,8,-12,-2,-4,-10,0,-12,-4,-6,-8,-6,8,-14,-2,-2,-2,-2,-2,-2,-2,-2,-2,-2,10
%N A140875 Triangle read by rows: T(n,m) = gcd(n,m+2) - 2*gcd(n,m+1) - gcd(n,m), with diagonal and subdiagonal removed.
%e A140875 { -2},
%e A140875 { -4, 0},
%e A140875 { -4, -4, 0},
%e A140875 { -6, -2, -2, 2},
%e A140875 { -6, -2, -6, -6, 2},
%e A140875 { -8, -2, -2, -2, -2, 4},
%e A140875 { -8, -4, 0, -8, -4, -4, 4},
%e A140875 {-10, 0, -6, -4, 0, -6, -4, 6},
%e A140875 {-10, -4, -2, 0, -10, -8, -2, -4, 6}
%t A140875 G[n_, m_] := GCD[n, m]
%t A140875 D2[n_, m_] := If[m + 2 <= n, G[n, m + 2] - 2*G[n, m + 1] - G[n, m], {} ];
%t A140875 a = Table[Flatten[Table[D2[n, m], {m, 0, n}]], {n, 0, 10}]; Flatten[a]
%Y A140875 Cf. A109004.
%K A140875 tabl,sign,easy,less
%O A140875 1,1
%A A140875 _Roger L. Bagula_ and _Gary W. Adamson_, Jul 21 2008
%E A140875 Definition corrected and more terms from _Georg Fischer_, Jun 08 2023