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A265483
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Numbers k such that 25^k - 5^k - 1 is prime.
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1
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1, 2, 4, 15, 16, 24, 57, 206, 284, 1290, 1722, 1862, 1866, 3271, 5306, 5474, 15401, 18729, 34757, 42842, 63930, 89967
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OFFSET
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1,2
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COMMENTS
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For k = 1, 2, 4, 15, 16, the corresponding primes are 19, 599, 389999, 931322574584960937499, 23283064365234374999999.
a(n) is not of the form 5*m + 3 (divisibility by 11) or 9*m + 10 (divisibility by 19), 7*m + (-1)^m + 7 (divisibility by 29) or 29*m + 27 (divisibility by 59).
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LINKS
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EXAMPLE
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4 is in the sequence because 25^4-5^4-1 = 389999 is prime.
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MATHEMATICA
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Select[Range[6000], PrimeQ[25^# - 5^# - 1] &]
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PROG
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(Magma) [n: n in [0..300] | IsPrime(25^n-5^n-1)];
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CROSSREFS
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Cf. similar sequences listed in A265481.
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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