OFFSET
1,3
COMMENTS
Conjecture 1: a(n) > 0 for all n > 1. In other words, any odd prime p has a primitive root g < p which is the sum of two central binomial coefficients.
Conjecture 2: Each odd prime p has a primitive root g < p which is the sum of two Catalan numbers.
We have verified Conjecture 1 for all odd primes p < 10^9.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..50000
Zhi-Wei Sun, New observations on primitive roots modulo primes, arXiv:1405.0290 [math.NT], 2014.
Zhi-Wei Sun, Problems on combinatorial properties of primes, in: M. Kaneko, S. Kanemitsu and J. Liu (eds.), Number Theory: Plowing and Starring through High Wave Forms, Proc. 7th China-Japan Seminar (Fukuoka, Oct. 28--Nov. 1, 2013), Ser. Number Theory Appl., Vol. 11, World Sci., Singapore, 2015, pp. 169-187.
EXAMPLE
a(2) = 1 with binomial(2*0,0) + binomial(2*0,0) = 2 a primitive root modulo prime(2) = 3.
a(3) = 2 with binomial(2*0,0) + binomial(2*0,0) = 2 and binomial(2*0,0) + binomial(2*1,1) = 3 primitive roots modulo prime(3) = 5.
a(4) = 1 with binomial(2*0,0) + binomial(2*1,1) = 3 a primitive root modulo prime(4) = 7.
a(29) = 2 with binomial(2*3,3) + binomial(2*3,3) = 40 and binomial(2*1,1) + binomial(2*4,4) = 72 primitive roots modulo prime(29) = 109.
MATHEMATICA
p[n_]:=p[n]=Prime[n];
Dv[n_]:=Dv[n]=Divisors[n];
gp[g_, p_]:=gp[g, p]=Mod[g, p]>0&&Sum[Boole[PowerMod[g, Dv[p-1][[k]], p]==1], {k, 1, Length[Dv[p-1]]-1}]==0;
tab={}; Do[r=0; a=0; Label[aa]; If[Binomial[2a, a]>=p[n], Goto[cc]]; b=0; Label[bb]; If[b>a||Binomial[2b, b]>=p[n]-Binomial[2a, a], a=a+1; Goto[aa]];
If[gp[Binomial[2a, a]+Binomial[2b, b], p[n]], r=r+1]; b=b+1; Goto[bb]; Label[cc]; tab=Append[tab, r], {n, 1, 90}]; Print[tab]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, May 24 2018
STATUS
approved