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Search: id:A097443
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| A097443 |
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Half-period primes, i.e. primes p for which reciprocal has period (p-1)/2. |
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+0
6
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| 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; listen)
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OFFSET
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1,1
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LINKS
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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
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EXAMPLE
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13 is a half-period prime because 1/13 = 0.076923076923076923076923... i.e. has period 6, or (13-1)/2.
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MATHEMATICA
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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 &] (from Robert G. Wilson v Sep 14 2004)
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CROSSREFS
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Cf. A002371, A001913. Almost the same as A001914.
Cf. A001913, A055628, A056157, A056210-A056217, A098680
Sequence in context: A023274 A129864 A080387 this_sequence A100589 A111489 A106294
Adjacent sequences: A097440 A097441 A097442 this_sequence A097444 A097445 A097446
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KEYWORD
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nonn
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AUTHOR
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Julien Peter Benney (jpbenney(AT)ftml.net), Aug 23 2004
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