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A000864
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Deceptive nonprimes: composite numbers k that divide the repunit R_{k-1}.
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3
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91, 259, 451, 481, 703, 1729, 2821, 2981, 3367, 4141, 4187, 5461, 6533, 6541, 6601, 7471, 7777, 8149, 8401, 8911, 10001, 11111, 12403, 13981, 14701, 14911, 15211, 15841, 19201, 21931, 22321, 24013, 24661, 27613, 29341, 34133
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OFFSET
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1,1
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COMMENTS
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Francis and Ray call these numbers "deceptive primes".
Pseudoprimes to base 10, A005939, not divisible by 3. If k is in the sequence, then (10^k-1)/9 is in the sequence, by Steuerwald's theorem; see A005935. - Thomas Ordowski, Apr 10 2016
41041 is the first term that has four prime divisors. - Altug Alkan, Apr 10 2016
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LINKS
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MAPLE
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select(t -> not isprime(t) and (10&^(t-1) - 1) mod (9*t) = 0, [seq(t, t=3..10^5, 2)]); # Robert Israel, Apr 10 2016
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PROG
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(PARI) p=5; forprime(q=7, 1e5, forstep(n=p+2, q-2, 2, if(n%5 && Mod(10, 9*n)^(n-1)==1, print1(n", "))); p=q) \\ Charles R Greathouse IV, Jul 31 2011
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Tim Ray (c268scm(AT)semovm.semo.edu)
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STATUS
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approved
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