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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 (list; graph; refs; listen; history; text; internal format)
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: A228492 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

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Last modified May 19 08:53 EDT 2019. Contains 323389 sequences. (Running on oeis4.)