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%I #22 Jun 29 2021 05:54:32
%S 11,251,1061,1451,1901,1931,2381,3181,3491,3851,4621,4861,5261,6101,
%T 6491,6581,6781,7331,8101,9941,10331,10771,11251,11261,11411,12301,
%U 14051,14221,14411,15091,15131,16061,16141,16301,16651,16811,16901
%N Primes p whose period of reciprocal equals (p-1)/5.
%C Cyclic numbers of the fifth degree (or fifth order): the reciprocals of these numbers belong to one of five different cycles. Each cycle has the (number minus 1)/5 digits.
%C From _Robert Israel_, Apr 02 2018: (Start)
%C Primes p such that A002371(A000720(p)) = (p-1)/5.
%C All terms == 1 (mod 10). (End)
%H Amiram Eldar, <a href="/A056210/b056210.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from T. D. Noe)
%H <a href="/index/1#1overn">Index entries for sequences related to decimal expansion of 1/n</a>
%p select(t -> isprime(t) and numtheory:-order(10, t) = (t-1)/5, [seq(t,t=11..17000,10)]); # _Robert Israel_, Apr 02 2018
%t f[n_Integer] := Block[{ds = Divisors[n - 1]}, (n - 1)/Take[ ds, Position[ PowerMod[ 10, ds, n], 1] [[1, 1]]] [[ -1]]]; Select[ Prime[ Range[4, 2000]], f[ # ] == 5 &]
%Y Cf. A000720, A001913, A002371, A097443, A055628, A056157, A056211, A056212, A056213, A056214, A056215, A056216, A056217, A098680.
%K nonn,base
%O 1,1
%A _Robert G. Wilson v_, Aug 02 2000
%E Entry revised by _N. J. A. Sloane_, Apr 30 2007
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