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A128356
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Least number k > 1 (that is not the power of prime p) such that k divides (p+1)^k-1, where p = prime(n).
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9
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20, 21, 1555, 889, 253, 2041, 5846759, 148305659, 1081, 279241, 9641, 950123, 33661, 63213709997, 583223, 3775349, 72707647, 149070763, 196932497, 5091481, 25760459, 14307947980741, 13861, 9362711, 376457, 132766545553, 63757
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OFFSET
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1,1
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COMMENTS
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All listed terms have 2 distinct prime divisors. Most listed terms are semiprimes, except a(7) = 20231*17^2 and a(8) = 410819*19^2. p = prime(n) divides a(n). Quotients a(n)/prime(n) are listed in A128357 = {10, 7, 311, 127, 23, 157, 343927, ...}. a(15) = 583223 = 47*12409. a(16) = 3775349 = 53*71233.
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LINKS
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MATHEMATICA
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(* This program is not suitable to compute a large number of terms *) a[n_] := For[p = Prime[n]; k = 2, True, k++, If[Length[FactorInteger[k]] == 2, If[Mod[PowerMod[p + 1, k, k] - 1, k] == 0, Print[k]; Return[k]]]]; Table[a[n], {n, 1, 13}] (* Jean-François Alcover, Oct 07 2013 *)
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CROSSREFS
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KEYWORD
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hard,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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