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 A291776 #14 Sep 01 2017 05:03:06
%S A291776 3,7,23,31,103,127,431,911,1847,6719,10487,8191,56999,41399,135647,
%T A291776 131071,560159,524287,1999871,3982271,5909759,17512991,46092239,
%U A291776 46335599,164460119,186592247,736727807,3926707199,4146049487,2147483647,8994904463,11132323439
%N A291776 a(n) = prime that is eventually reached when x -> sigma(x)-1 is repeatedly applied to 2^n-1, or -1 if no prime is ever reached.
%H A291776 Lars Blomberg, <a href="/A291776/b291776.txt">Table of n, a(n) for n = 2..270</a>
%e A291776 For n=9, 2^n-1 = 511 with iterates 511->591->791->911, and 911 is the first prime, so a(7)=911.
%o A291776 (PARI) P(x) = {for(c=0,10^6,if(isprime(x),return(x),x=sigma(x)-1));-1}
%o A291776 vector(200,n,P(2^(n+1)-1)) \\ _Lars Blomberg_, Sep 01 2017
%Y A291776 Cf. A039654, A039655, A291301, A291302, A291777.
%K A291776 nonn
%O A291776 2,1
%A A291776 _N. J. A. Sloane_, Aug 31 2017
%E A291776 Added a(7) and a(13)-a(33) from _Lars Blomberg_, Sep 01 2017