OFFSET
1,1
COMMENTS
It is well known that for any prime p the number of distinct primitive roots modulo p among 1,...,p-1 is phi(p-1).
Conjecture: The sequence contains infinitely many terms. Moreover, the number of primes p <= x with phi(p-1) a primitive root modulo p is asymptotically equivalent to c*x/(log x) as x tends to the infinity, where c is a constant with 0.36 < c < 0.37.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..10000
Zhi-Wei Sun, New observations on primitive roots modulo primes, arXiv:1405.0290 [math.NT], 2014.
EXAMPLE
a(2) = 5 since phi(5-1) = 2 is a primitive root modulo the prime 5.
MATHEMATICA
p[n_]:=p[n]=Prime[n];
n=0; Do[Do[If[Mod[EulerPhi[p[k]-1]^(Part[Divisors[p[k]-1], i])-1, p[k]]==0, Goto[aa]], {i, 1, Length[Divisors[p[k]-1]]-1}];
n=n+1; Print[n, " ", p[k]]; Label[aa], {k, 1, 220}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Oct 02 2017
STATUS
approved