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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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..2000 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 Adjacent sequences:  A180260 A180261 A180262 * A180264 A180265 A180266 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

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Last modified May 11 21:35 EDT 2021. Contains 343808 sequences. (Running on oeis4.)