cp's OEIS Frontend

A291777 a(n) = number of steps to reach a prime when x -> sigma(x)-1 is repeatedly applied to 2^n-1, or -1 if no prime is ever reached.

%I A291777 #11 Sep 01 2017 05:03:18
%S A291777 0,0,1,0,1,0,1,3,2,9,2,0,7,3,4,0,2,0,1,4,1,4,2,3,4,2,12,22,8,0,3,3,4,
%T A291777 3,1,2,2,3,3,4,3,13,2,16,3,8,3,14,17,9,37,4,7,4,7,11,4,3,14,0,14,8,1,
%U A291777 6,8,73,26,10,1,32,6,10,2,6,2,33,2,4,52,12,16
%N A291777 a(n) = number of steps to reach a prime when x -> sigma(x)-1 is repeatedly applied to 2^n-1, or -1 if no prime is ever reached.
%H A291777 Lars Blomberg, <a href="/A291777/b291777.txt">Table of n, a(n) for n = 2..270</a>
%e A291777 For n=9, 2^n-1 = 511 with iterates 511->591->791->911, and 911 is the first prime, so a(7)=3.
%o A291777 (PARI) C(x) = {for(c=0,10^5,if(isprime(x),return(c),x=sigma(x)-1));-1}
%o A291777 vector(200,n,C(2^(n+1)-1)) \\ _Lars Blomberg_, Sep 01 2017
%Y A291777 Cf. A039654, A039655, A291301, A291302, A291776.
%K A291777 nonn
%O A291777 2,8
%A A291777 _N. J. A. Sloane_, Aug 31 2017
%E A291777 a(13)-a(82) from _Lars Blomberg_, Sep 01 2017