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 A084307 #37 Oct 29 2019 08:59:48
%S A084307 1,13,17,6,199,5,242,27,391,57,1296,22,882,12,648,93,175232,89,3872,
%T A084307 236,195,1032,4875263,14,5739271,467,35377,83,1882384,58,3024,308,
%U A084307 29240,201,1627208,118,79524,147,1682,56,46854024,82,229441,1204,2047,6301,83386957823
%N A084307 a(n) is the least number x such that gcd(sigma(x), sigma(x+1)) = n.
%C A084307 a(47) > 10^9 if it exists. - _Michel Marcus_, Sep 01 2019
%e A084307 n=9: GCD[sigma[x+1], sigma[x]]=5 holds for {391,799,800,881,...} of which the first is a(9)=391.
%t A084307 f[x_] := GCD[DivisorSigma[1, x], DivisorSigma[1, x+1]] t=Table[0, {256}]; Do[s=f[n]; If[s<257&&t[[s]]==0, t[[s]]=n], {n, 1, 10000000}]; t
%o A084307 (PARI) a(n) = my(x=1, sx=sigma(x), y=2, sy=sigma(2)); while(gcd(sx, sy) != n, x++; y++; sx=sy; sy=sigma(y)); x; \\ _Michel Marcus_, Aug 28 2019
%Y A084307 Cf. A000203, A060780, A063444.
%K A084307 nonn
%O A084307 1,2
%A A084307 _Labos Elemer_, Jun 13 2003
%E A084307 a(41) from _Rémy Sigrist_, Aug 29 2019
%E A084307 a(42)-a(46) from _Michel Marcus_, Aug 30 2019
%E A084307 a(47) from _Giovanni Resta_, Oct 29 2019