OFFSET
1,11
COMMENTS
A prime factor of 2^n - 1 is called primitive if it does not divide 2^r - 1 for any r < n. Equivalently, p is a primitive prime factor of 2^n - 1 if ord(2,p) = n. Zsigmondy's theorem says that there is at least one primitive prime factor for n > 1, except for n=6. See A086252 for those n that have a record number of primitive prime factors.
Number of odd primes p such that A002326((p-1)/2) = n. Number of occurrences of number n in A014664. - Thomas Ordowski, Sep 12 2017
The prime factors are not counted with multiplicity, which matters for a(364)=4 and a(1755)=6. - Jeppe Stig Nielsen, Sep 01 2020
LINKS
Eric Weisstein's World of Mathematics, Zsigmondy Theorem
FORMULA
a(n) <= A182590(n). - Thomas Ordowski, Sep 14 2017
EXAMPLE
a(11) = 2 because 2^11 - 1 = 23*89 and both 23 and 89 have order 11.
MATHEMATICA
Join[{0}, Table[cnt=0; f=Transpose[FactorInteger[2^n-1]][[1]]; Do[If[MultiplicativeOrder[2, f[[i]]]==n, cnt++ ], {i, Length[f]}]; cnt, {n, 2, 200}]]
PROG
(PARI) a(n) = sumdiv(n, d, moebius(n/d)*omega(2^d-1)); \\ Michel Marcus, Sep 12 2017
(PARI) a(n) = my(m=polcyclo(n, 2)); omega(m/gcd(m, n)) \\ Jeppe Stig Nielsen, Sep 01 2020
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
T. D. Noe, Jul 14 2003
EXTENSIONS
Terms to a(500) in b-file from T. D. Noe, Nov 11 2010
Terms a(501)-a(1200) in b-file from Charles R Greathouse IV, Sep 14 2017
Terms a(1201)-a(1206) in b-file from Max Alekseyev, Sep 11 2022
STATUS
approved