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A056619
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Smallest prime with primitive root n, or 0 if no such prime exists.
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5
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2, 3, 2, 0, 2, 11, 2, 3, 2, 7, 2, 5, 2, 3, 2, 0, 2, 5, 2, 3, 2, 5, 2, 7, 2, 3, 2, 5, 2, 11, 2, 3, 2, 19, 2, 0, 2, 3, 2, 7, 2, 5, 2, 3, 2, 11, 2, 5, 2, 3, 2, 5, 2, 7, 2, 3, 2, 5, 2, 19, 2, 3, 2, 0, 2, 7, 2, 3, 2, 19, 2, 5, 2, 3, 2, 13, 2, 5, 2, 3, 2, 5, 2, 11, 2, 3, 2, 5, 2, 11, 2, 3, 2, 7, 2, 7, 2, 3, 2
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OFFSET
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1,1
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COMMENTS
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a(n) > n/2 for n in { 2, 6, 10, 34 }. Are there any other such indices n? - M. F. Hasler, Feb 21 2017
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LINKS
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FORMULA
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a(n) = 0 only for perfect squares, A000290.
a(n) = 2 for all odd n. a(n) = 0 for even squares. a(n) = 3 for n = 2 (mod 6). a(n) = 5 for n in {12, 18, 22, 28} (mod 30). - M. F. Hasler, Feb 21 2017
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MAPLE
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f:= proc(n) local p;
if n::odd then return 2
elif issqr(n) then return 0
fi;
p:= 3;
do
if numtheory:-order(n, p) = p-1 then return p fi;
p:= nextprime(p);
od
end proc:
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MATHEMATICA
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a[n_] := Module[{p}, If[OddQ[n], Return[2], If[IntegerQ[Sqrt[n]], Return[0], p = 3; While[True, If[MultiplicativeOrder[n, p] == p-1, Return[p]]; p = NextPrime[p]]]]];
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PROG
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(PARI) A056619(n)=forprime(p=2, n*2, gcd(n, p)==1&&znorder(Mod(n, p))==p-1&&return(p)) \\ or, more efficient:
A056619(n)=if(bittest(n, 0), 2, !issquare(n)&&forprime(p=3, n*2, gcd(n, p)==1&&znorder(Mod(n, p))==p-1&&return(p))) \\ M. F. Hasler, Feb 21 2017
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CROSSREFS
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Here the primitive root may be larger than the prime, whereas in A023049 it may not be.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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