OFFSET
1,1
COMMENTS
Equivalently, p-1 divided by the period of the decimal expansion of 1/p equals 2^x for some nonnegative integer x. Composite numbers satisfying this condition are given in A243050. - Lear Young, May 30 2013
Let pi_1(x) and pi(x) be the numbers of primes of this sequence and all primes not exceeding x, respectively. Then, for x>=3, p_1(x)/pi(x) >= C_Artin = 0.37395581... Numerical results suggest that it is likely lim pi_1(x)/pi(x) = 2*C_Artin. - Peter J. C. Moses and Vladimir Shevelev, May 29 2014
LINKS
Lear Young, Table of n, a(n) for n = 1..1000
EXAMPLE
(41-1)/20 = 2. 20 is the period of the binary representation of 1/n, the odd part of 2 is 1.
MATHEMATICA
Select[Prime[Range[2, 100]], # == 2^IntegerExponent[#, 2] &[(# - 1)/MultiplicativeOrder[2, #]] &] (* Peter J. C. Moses, May 28 2014 *)
PROG
(PARI)
is(n) = {
m = valuation(n+1, 2);
k=(n+1)>>m;
if(k!=1, for(i=0, (n-1)>>1,
l=valuation(k+n, 2);
k=(k+n)>>l;
m+=l; if(k==1, break)));
((n-1)/m)>>valuation((n-1)/m, 2)==1
\\ m equals znorder(Mod(2, n))
}
forstep(i=3, 1e3, 2, if(is(i), print1(i, ", ")))
\\ Lear Young May 30 2013
(PARI)
forstep(i=1, 1e3, 2, j = (i-1)/znorder(Mod(2, i)); if(j>>valuation(j, 2)==1, print1(i, ", "))) \\ Lear Young May 31 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lear Young, May 30 2013
STATUS
approved