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Search: id:A128164
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| A128164 |
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Least k>2 such that (n^k-1)/(n-1) is prime, or 0 if no such prime exists. |
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3
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| 3, 3, 0, 3, 3, 5, 3, 0, 19, 17, 3, 5, 3, 3, 0, 3, 25667, 19, 3, 3, 5, 5, 3, 0, 7, 3, 5, 5, 5, 7, 0, 3, 13, 313, 0, 13, 3, 349, 5, 3, 1319, 5, 5, 19, 7, 127, 19, 0, 3, 4229, 103, 11, 3, 17, 7, 3, 41, 3, 7, 7, 3, 5, 0, 19, 3, 19, 5, 3, 29, 3, 7, 5, 5, 3, 41, 3, 3, 5, 3, 0, 23, 5, 17, 5, 11, 7, 61, 3, 3
(list; graph; listen)
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OFFSET
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2,1
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COMMENT
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a(n) = A084740(n) for all n except n = p-1, where p is an odd prime, for which A084740(n) = 2.
All nonzero terms are odd primes.
a(n) = 0 for n = {4,9,16,25,32,36,49,64,81,100,121,125,144,...}, which are the perfect powers with exceptions of the form n^(p^m) where p>2 and (n^(p^(m+1))-1)/(n^(p^m)-1) are prime and m>=1 (in which case a(n^(p^m))=p). - Max Alekseyev, Jan 24 2009
a(n) = 3 for n in A002384, i.e. for n such that n^2 + n + 1 is prime.
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REFERENCES
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H. Dubner, Generalized repunit primes, Math. Comp., 61 (1993), 927-930.
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LINKS
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Max Alekseyev, Table of n, a(n) for n = 2..151
Eric Weisstein's World of Mathematics, Repunit
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CROSSREFS
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Cf. A084738, A065854, A084740, A084741, A065507, A084742
Sequence in context: A084055 A084103 A036477 this_sequence A140686 A116580 A096439
Adjacent sequences: A128161 A128162 A128163 this_sequence A128165 A128166 A128167
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KEYWORD
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nonn
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AUTHOR
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Alexander Adamchuk (alex(AT)kolmogorov.com), Feb 20 2007
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EXTENSIONS
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a(18) = 25667 found by Henri Lifchitz, Sep 26 2007
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