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A180263
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Odd k such that (k^2 + 1)/2 is not prime.
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2
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1, 7, 13, 17, 21, 23, 27, 31, 33, 37, 41, 43, 47, 53, 55, 57, 63, 67, 73, 75, 77, 81, 83, 87, 89, 91, 93, 97, 99, 103, 105, 107, 109, 111, 113, 115, 117, 119, 123, 125, 127, 129, 133, 135, 137, 143, 147, 149, 151, 153, 155, 157, 161, 163, 167, 173, 177, 179, 183, 185
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OFFSET
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1,2
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LINKS
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EXAMPLE
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a(2)=7: (7^2 + 1)/2 = 25, which is not prime, so 7 is in the sequence.
(9^2 + 1)/2 = 41, which is prime, so 9 is not in the sequence.
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MATHEMATICA
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a={}; For[i=1, i<100, i=i+2, If[PrimeQ[(i^2+1)/2], 0, AppendTo[a, i]]]Print[a]
Select[Range[1, 201, 2], !PrimeQ[(#^2+1)/2]&] (* Harvey P. Dale, Jan 07 2016 *)
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PROG
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(Magma) [ n: n in [1..200 by 2] | not IsPrime((n^2+1) div 2) ];
(PARI) isok(n) = (n%2) && !isprime((n^2 + 1)/2); \\ Michel Marcus, Nov 23 2018
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CROSSREFS
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Cf. A027862 (primes of the form (n^2+1)/2).
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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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