A208645 Least x>0 such that x^2+x+n is not prime.

2, 4, 1, 2, 1, 4, 1, 1, 1, 2, 1, 10, 1, 1, 1, 2, 1, 16, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 1, 4, 1, 1, 1, 2, 1, 40, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 1, 3, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 1, 6, 1, 1, 1, 2, 1, 1, 1, 1

Offset

0,1

Comments

By definition, a(n)>0 for all n, and a(n)>1 if n+2 is prime.

Links

Charles R Greathouse IV, Table of n, a(n) for n = 0..10000

Example

a(0)=2 since 1^2+1+0=2 is prime, but 2^2+2+0=6 is composite.
a(1)=4 since 1^2+1+1=2, 2^2+2+1=7 and 3^2+3+1=13 are prime, but 4^2+4+1=21 is composite.

Mathematica

lx[n_]:=Module[{x=1}, While[PrimeQ[x^2+x+n], x++]; x]; Array[lx, 90, 0] (* Harvey P. Dale, Aug 14 2013 *)

Prog

(PARI) a(n)=for( x=1, n+3, isprime(x^2+x+n) | return(x))

Crossrefs

Cf. A208936, A014556, A092749.

Cf. A117530, A117531.

Sequence in context: A133902 A265823 A305717 * A361381 A054708 A112229

Adjacent sequences: A208642 A208643 A208644 * A208646 A208647 A208648

Keyword

nonn

Author

M. F. Hasler, Mar 03 2012

Status

approved