A180263 Odd k such that (k^2 + 1)/2 is not prime.

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

Author

Alex Meiburg, Aug 21 2010

Keywords

• nonn

Examples

			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.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..2000

Crossrefs

Cf. A027862 (primes of the form (n^2+1)/2).

Programs

Magma

[ n: n in [1..200 by 2] | not IsPrime((n^2+1) div 2) ];

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 *)

PARI

isok(n) = (n%2) && !isprime((n^2 + 1)/2); \\ Michel Marcus, Nov 23 2018

Extensions

More terms from Vincenzo Librandi, Nov 18 2010

Example clarified by Harvey P. Dale, Jan 07 2016