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.

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

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)=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

Cf. A039654, A039655, A291301, A291302, A291776.

Sequence in context: A329038 A272491 A010271 * A143074 A173624 A182023

Adjacent sequences: A291774 A291775 A291776 * A291778 A291779 A291780

Keyword

nonn

Author

N. J. A. Sloane, Aug 31 2017

Extensions

a(13)-a(82) from Lars Blomberg, Sep 01 2017

Status

approved