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, 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

Lars Blomberg, Table of n, a(n) for n = 2..270

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

Cf. A039654, A039655, A291301, A291302, A291777.

Sequence in context: A260350 A270384 A213897 * A135570 A053027 A133432

Adjacent sequences: A291773 A291774 A291775 * A291777 A291778 A291779

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