A243403 Number of primes p < n such that p*(n-p) is a primitive root modulo prime(n).

0, 0, 1, 1, 2, 0, 3, 2, 3, 2, 1, 3, 3, 2, 3, 4, 4, 1, 4, 1, 2, 2, 5, 8, 5, 1, 1, 5, 3, 6, 6, 7, 6, 6, 4, 2, 4, 3, 6, 11, 6, 4, 3, 7, 6, 8, 3, 2, 10, 9, 6, 11, 2, 8, 9, 9, 5, 2, 5, 2, 3, 13, 5, 14, 8, 12, 7, 8, 9, 6, 13, 9, 4, 10, 3, 13, 12, 4, 8, 4

Offset

1,5

Comments

Conjecture: a(n) > 0 for all n > 6.

We have verified this for all n = 7, ..., 2*10^5.

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(18) = 1 since 17 is prime with 17*(18-17) = 17 a primitive root modulo prime(18) = 61.
a(20) = 1 since 11 is prime with 11*(20-11) = 99 a primitive root modulo prime(20) = 71.
a(26) = 1 since 2 is prime with 2*(26-2) = 48 a primitive root modulo prime(26) = 101.
a(27) = 1 since 17 is prime with 17*(27-17) = 170 a primitive root modulo prime(27) = 103.

Mathematica

dv[n_]:=Divisors[n]
Do[m=0; Do[Do[If[Mod[(Prime[k]*(n-Prime[k]))^(Part[dv[Prime[n]-1], i]), Prime[n]]==1, Goto[aa]], {i, 1, Length[dv[Prime[n]-1]]-1}]; m=m+1; Label[aa]; Continue, {k, 1, PrimePi[n-1]}];
Print[n, " ", m]; Continue, {n, 1, 80}]

Crossrefs

Cf. A000040, A000720, A243164.

Sequence in context: A324397 A132623 A277890 * A051613 A173291 A341889

Adjacent sequences: A243400 A243401 A243402 * A243404 A243405 A243406

Keyword

nonn

Author

Zhi-Wei Sun, Jun 04 2014

Status

approved