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%I #6 Jul 04 2021 03:23:28
%S 5,38,387,3755,37523,374126,3740610,37393725,373953691,3739544360
%N Number of cyclic numbers, primes with primitive root 10, (A001913) in the first 10^n primes (A000040).
%F Lim_{n->oo} a(n)/10^n = Artin's constant (A005596).
%t f[n_Integer] := Block[{ds = Divisors[n - 1]}, (n - 1)/Take[ds, Position[ PowerMod[10, ds, n], 1][[1, 1]]][[ -1]]]; c = 0; k = 4; Do[ While[k <= 10^n, a = f[ Prime[k]]; If[a == 1, c++ ]; k++ ]; Print[c], {n, 7}]
%Y Cf. A001913, A006883, A000040, A005596.
%K base,more,nonn
%O 1,1
%A _Robert G. Wilson v_, Oct 19 2004
%E a(8)-a(10) from _Amiram Eldar_, Jul 04 2021