A134407 Numbers n such that n^2 + 1 is composite.

3, 5, 7, 8, 9, 11, 12, 13, 15, 17, 18, 19, 21, 22, 23, 25, 27, 28, 29, 30, 31, 32, 33, 34, 35, 37, 38, 39, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 55, 57, 58, 59, 60, 61, 62, 63, 64, 65, 67, 68, 69, 70, 71, 72, 73, 75, 76, 77, 78, 79, 80, 81, 82, 83, 85, 86, 87, 88

Offset: 1

Author

Jani Melik, Jan 18 2008

Keywords

• nonn
• easy

Examples

			a(1)=3, because 3^2 + 1 = 10 is composite,
a(2)=5, because 5^2 + 1 = 26 is composite,
a(3)=7, because 7^2 + 1 = 50 is composite.
		

Crossrefs

Cf. A002496, A002808, A005574, A018252.

Programs

Maple

ts_fn2:=proc(n) local i,tren,ans; ans:=[ ]: for i from 1 to n do tren := i^(2)+1: if (isprime(tren) = false) then ans:=[ op(ans), i ]: fi od: RETURN(ans) end: ts_fn2(200);

Mathematica

Select[Range@100,!PrimeQ[#^2+1]&] (* Vladimir Joseph Stephan Orlovsky, Feb 03 2012 *)

PARI

is(n)=!isprime(n^2+1) \\ Charles R Greathouse IV, Sep 15 2014

Formula

a(n) ~ n. - Charles R Greathouse IV, Sep 15 2014