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 A066715 #11 Jul 22 2019 16:24:35
%S A066715 1,1,1,1,1,1,1,3,1,1,1,1,1,1,1,1,3,1,1,1,1,1,3,1,1,3,1,1,1,1,1,1,1,1,
%T A066715 3,1,1,1,1,1,1,1,1,3,1,7,1,5,1,3,1,1,3,1,1,1,1,1,13,1,1,3,1,1,1,1,1,
%U A066715 15,1,1,3,1,5,3,1,1,9,1,1,3,1,1,3,1,1,1,1,1,3,1,1,1,1,1,1,1,1,3,1,1,1,1,1,3
%N A066715 a(n) = gcd(2n+1, sigma(2n+1)).
%C A066715 If gcd(n, sigma(n))=1 then n is an odd perfect number. It seems however that gcd(n, sigma(n)) is always significantly less than n.
%H A066715 Harry J. Smith, <a href="/A066715/b066715.txt">Table of n, a(n) for n = 0..1000</a>
%e A066715 a(5) = 1 as gcd(5,6) = 1. a(15) = gcd(15, sigma(15)) = gcd(15,(1+3+5+15)) = gcd(15,24) = 3.
%t A066715 Table[GCD[2n+1,DivisorSigma[1,2n+1]],{n,0,120}] (* _Harvey P. Dale_, Jul 22 2019 *)
%o A066715 (PARI) forstep (x=3,2000,2,write1("oddperfectgcd.txt",gcd(sigma(x),x),","))
%o A066715 (PARI) { for (n=0, 1000, write("b066715.txt", n, " ", gcd(2*n+1, sigma(2*n+1))) ) } \\ _Harry J. Smith_, Mar 19 2010
%K A066715 nonn
%O A066715 0,8
%A A066715 _Jon Perry_, Jan 14 2002