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A180263
Odd k such that (k^2 + 1)/2 is not prime.
2
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
OFFSET
1,2
LINKS
EXAMPLE
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.
MATHEMATICA
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 *)
PROG
(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
CROSSREFS
Cf. A027862 (primes of the form (n^2+1)/2).
Sequence in context: A276045 A230039 A226138 * A002733 A108334 A288713
KEYWORD
nonn
AUTHOR
Alex Meiburg, Aug 21 2010
EXTENSIONS
More terms from Vincenzo Librandi, Nov 18 2010
Example clarified by Harvey P. Dale, Jan 07 2016
STATUS
approved