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A208645 Least x>0 such that x^2+x+n is not prime. 1
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 (list; graph; refs; listen; history; text; internal format)
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 * A054708 A112229 A087419

Adjacent sequences:  A208642 A208643 A208644 * A208646 A208647 A208648

KEYWORD

nonn

AUTHOR

M. F. Hasler, Mar 03 2012

STATUS

approved

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Last modified April 17 02:26 EDT 2021. Contains 343059 sequences. (Running on oeis4.)