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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.
3
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, 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, 6, 8, 73, 26, 10, 1, 32, 6, 10, 2, 6, 2, 33, 2, 4, 52, 12, 16
OFFSET
2,8
LINKS
EXAMPLE
For n=9, 2^n-1 = 511 with iterates 511->591->791->911, and 911 is the first prime, so a(7)=3.
PROG
(PARI) C(x) = {for(c=0, 10^5, if(isprime(x), return(c), x=sigma(x)-1)); -1}
vector(200, n, C(2^(n+1)-1)) \\ Lars Blomberg, Sep 01 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Aug 31 2017
EXTENSIONS
a(13)-a(82) from Lars Blomberg, Sep 01 2017
STATUS
approved