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A056561
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Numbers n such that n^2 + n + 41 is prime.
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23
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0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 42, 43, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 57, 58, 59, 60, 61, 62, 63, 64, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 77, 78
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OFFSET
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1,3
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COMMENTS
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Among first 100000 terms, the only run of 13 subsequent values >39 is 219..231. - Zak Seidov, Jan 28 2009
Number of terms less than 10^n: 1, 10, 86, 581, 4149, 31985, 261081, 2208197, 19132652, ... . - Robert G. Wilson v, Apr 20 2015
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REFERENCES
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P. Hoffman, Archimedes' Revenge, pp. 39-40,Penguin Books 1988.
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LINKS
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FORMULA
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a(n) = (sqrt(4*A005846(n)-163)-1)/2.
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EXAMPLE
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39 is in the sequence because 39^2+39+41=1601 which is prime but 40 is not because 40^2+40+41=1681=41*41.
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MAPLE
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select(t -> isprime(t^2+t+41), [$0..100]); # Robert Israel, Apr 20 2015
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MATHEMATICA
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PROG
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(Haskell)
a056561 n = a056561_list !! (n-1)
a056561_list = filter ((== 1) . a010051' . a202018) [0..]
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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