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A093324 a(n) is the smallest natural number m such that n^m + m is prime. 5
2, 1, 1, 2, 1, 7954, 1, 34, 101, 2, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

a(n)=1 iff n+1 is prime. For n>1 n+a(n) is odd (for n>1 a(n) is odd iff n is even). a(11) is greater than 7000. a(m) for m=12,13,14,15,16,17 and 18 are 1, 54, 17, 2, 1, 1080, 1. 5^7954 + 7954 is a 5560-digit probable prime.

a(11) is greater than 8651. - Robert G. Wilson v, May 24 2004

a(11) is greater than 20165, so every prime of the form 11^m + m has more than 21000 digits. - Farideh Firoozbakht, Jun 10 2004

a(11) is greater than 80000. - T. D. Noe, Mar 07 2007

a(11) is greater than 190000. - T. D. Noe, Jul 31 2008

LINKS

Table of n, a(n) for n=0..10.

FORMULA

a[n_]:=(For[m=1, !PrimeQ[n^m+m], m++ ]; m).

EXAMPLE

a(3)=2 because 3^2 + 2 is prime and 3^1 + 1 is composite.

MATHEMATICA

a[n_]:=(For[m=1, !PrimeQ[n^m+m], m++ ]; m); Do[Print[a[n]], {n, 0, 10}]

CROSSREFS

Sequence in context: A230324 A060256 A103899 * A169676 A034798 A225803

Adjacent sequences:  A093321 A093322 A093323 * A093325 A093326 A093327

KEYWORD

more,nonn

AUTHOR

Farideh Firoozbakht, May 11 2004

STATUS

approved

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Last modified July 21 06:52 EDT 2019. Contains 325191 sequences. (Running on oeis4.)