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A316907
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Numbers k such that 2^(k-1) == 1 (mod k) and p-1 does not divide k-1 for every prime p dividing k.
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4
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7957, 23377, 35333, 42799, 49981, 60787, 129889, 150851, 162193, 164737, 241001, 249841, 253241, 256999, 280601, 318361, 452051, 481573, 556169, 580337, 617093, 665333, 722201, 838861, 877099, 1016801, 1251949, 1252697, 1325843, 1507963, 1534541, 1678541, 1826203, 1969417
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OFFSET
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1,1
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COMMENTS
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Numbers k such that 2^k == 2 (mod k) and gcd(k,b^k-b) = 1 for some b > 2.
Problem: are there infinitely many such "anti-Carmichael pseudoprimes"?
All semiprime terms of A316906 are in the sequence; i.e., numbers m in A214305 such that p-1 does not divide q-1, where m = pq and p < q are primes.
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LINKS
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EXAMPLE
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7957 = 73*109 is pseudoprime and 72 does not divide 7956 (of course also 108 does not divide 7956), note that 72 does not divide 108.
617093 = 43*113*127 is the smallest such pseudoprime with more than two prime factors.
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MATHEMATICA
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Select[Select[Range[2*10^6], PowerMod[2, # - 1, #] == 1 &], Function[k, AllTrue[FactorInteger[k][[All, 1]] - 1, Mod[k - 1, #] != 0 &]]] (* Michael De Vlieger, Jul 20 2018 *)
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CROSSREFS
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Cf. A121707 (probably "anti-Carmichael numbers": n such that p-1 does not divide n-1 for every prime p dividing n).
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KEYWORD
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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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