A056210 - OEIS

A056210

Primes p whose period of reciprocal equals (p-1)/5.

11, 251, 1061, 1451, 1901, 1931, 2381, 3181, 3491, 3851, 4621, 4861, 5261, 6101, 6491, 6581, 6781, 7331, 8101, 9941, 10331, 10771, 11251, 11261, 11411, 12301, 14051, 14221, 14411, 15091, 15131, 16061, 16141, 16301, 16651, 16811, 16901

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OFFSET

1,1

COMMENTS

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.

From Robert Israel, Apr 02 2018: (Start)

Primes p such that A002371(A000720(p)) = (p-1)/5.

All terms == 1 (mod 10). (End)

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from T. D. Noe)

Index entries for sequences related to decimal expansion of 1/n

MAPLE

select(t -> isprime(t) and numtheory:-order(10, t) = (t-1)/5, [seq(t, t=11..17000, 10)]); # Robert Israel, Apr 02 2018

MATHEMATICA

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 &]

CROSSREFS

Cf. A000720, A001913, A002371, A097443, A055628, A056157, A056211, A056212, A056213, A056214, A056215, A056216, A056217, A098680.

Sequence in context: A377328 A238751 A098672 * A182350 A190680 A377327

Adjacent sequences: A056207 A056208 A056209 * A056211 A056212 A056213

KEYWORD

nonn,base

AUTHOR

Robert G. Wilson v, Aug 02 2000

EXTENSIONS

Entry revised by N. J. A. Sloane, Apr 30 2007

STATUS

approved