

A291777


a(n) = number of steps to reach a prime when x > sigma(x)1 is repeatedly applied to 2^n1, 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^n1 = 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



