

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
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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



