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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.
3
3, 7, 23, 31, 103, 127, 431, 911, 1847, 6719, 10487, 8191, 56999, 41399, 135647, 131071, 560159, 524287, 1999871, 3982271, 5909759, 17512991, 46092239, 46335599, 164460119, 186592247, 736727807, 3926707199, 4146049487, 2147483647, 8994904463, 11132323439
OFFSET
2,1
LINKS
EXAMPLE
For n=9, 2^n-1 = 511 with iterates 511->591->791->911, and 911 is the first prime, so a(7)=911.
PROG
(PARI) P(x) = {for(c=0, 10^6, if(isprime(x), return(x), x=sigma(x)-1)); -1}
vector(200, n, P(2^(n+1)-1)) \\ Lars Blomberg, Sep 01 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Aug 31 2017
EXTENSIONS
Added a(7) and a(13)-a(33) from Lars Blomberg, Sep 01 2017
STATUS
approved