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A097443 Half-period primes, i.e., primes p for which reciprocal has period (p-1)/2. 6
13, 31, 43, 67, 71, 83, 89, 107, 151, 157, 163, 191, 197, 199, 227, 283, 293, 307, 311, 347, 359, 373, 401, 409, 431, 439, 443, 467, 479, 523, 557, 563, 569, 587, 599, 601, 631, 653, 677, 683, 719, 761, 787, 827, 839, 877, 881, 883, 911, 919, 929, 947, 991 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

Makoto Kamada, Factorizations of 11...11 (Repunit).

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

EXAMPLE

13 is a half-period prime because 1/13 = 0.076923076923076923076923... i.e., has period 6, or (13-1)/2.

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, 200]], f[ # ] == 2 &] (* Robert G. Wilson v, Sep 14 2004 *)

CROSSREFS

Cf. A002371, A001913. Almost the same as A001914.

Cf. A001913, A055628, A056157, A056210-A056217, A098680

Sequence in context: A023274 A129864 A080387 * A100589 A111489 A106294

Adjacent sequences:  A097440 A097441 A097442 * A097444 A097445 A097446

KEYWORD

nonn

AUTHOR

Julien Peter Benney (jpbenney(AT)ftml.net), Aug 23 2004

STATUS

approved

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Last modified August 14 13:36 EDT 2018. Contains 313751 sequences. (Running on oeis4.)