OFFSET
1,1
COMMENTS
Number of iterations x->2x+1 needed to get a composite number, when starting with prime(n).
prime(n) is in A005384, i.e., a Sophie Germain prime, iff a(n)>1.
a(n) is the least k such that 2^k * (prime(n)+1) - 1 is composite. Note that a(n) is well defined since 2^(p-1) * (p+1) - 1 is divisible by p for odd primes p. - Jianing Song, Nov 24 2021
LINKS
T. D. Noe, Table of n, a(n) for n = 1..10000
G. Löh, Long chains of nearly doubled primes, Math. Comp., 53 (1989), 751-759.
Wikipedia, Cunningham chain
FORMULA
a(n) < prime(n) for n > 1; see Löh (1989), p. 751. - Jonathan Sondow, Oct 28 2015
EXAMPLE
2 -> 5 -> 11 -> 23 -> 47 -> 95 = 5*19, so a(1) = 5, a(3) = 4, a(5) = 3, a(9) = 2, and a(15) = 1. - Jonathan Sondow, Oct 30 2015
MATHEMATICA
Table[p = Prime[n]; cnt = 1; While[p = 2*p + 1; PrimeQ[p], cnt++]; cnt, {n, 100}] (* T. D. Noe, Jul 12 2012 *)
PROG
(PARI) a(n)= n=prime(n); for(c=1, 1e9, is/*pseudo*/prime(n=2*n+1) || return(c))
CROSSREFS
KEYWORD
nonn
AUTHOR
M. F. Hasler, Nov 17 2010
EXTENSIONS
Definition clarified by Jonathan Sondow, Oct 28 2015
STATUS
approved