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A092067 a(n) is the smallest number m such that m>1 and m divides n^m+1. 2
2, 3, 2, 5, 2, 7, 2, 3, 2, 11, 2, 13, 2, 3, 2, 17, 2, 19, 2, 3, 2, 23, 2, 5, 2, 3, 2, 29, 2, 31, 2, 3, 2, 5, 2, 37, 2, 3, 2, 41, 2, 43, 2, 3, 2, 47, 2, 7, 2, 3, 2, 53, 2, 5, 2, 3, 2, 59, 2, 61, 2, 3, 2, 5, 2, 67, 2, 3, 2, 71, 2, 73, 2, 3, 2, 7, 2, 79, 2, 3, 2, 83, 2, 5, 2, 3, 2, 89, 2, 7, 2, 3, 2, 5, 2, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n)=2 iff n is odd. If n is even then every prime factor of n+1 is a solution of the equation Mod[n^x+1,x]=0 and if n is odd, smallest prime factor of n+1 (2) is a solution of Mod[n^x+1,x]=0, so for each n, a(n) is not greater than the smallest prime factor of n+1. Conjecture 1: All terms of this sequence are primes. We know if n is odd a(n) is the smallest prime factor of n+1.Conjecture 2: For each n, a(n) is the smallest prime factor of n+1 or a(n)=A020639(n+1).

LINKS

Indranil Ghosh, Table of n, a(n) for n = 1..10000

FORMULA

a[n_] := (For[k=2, Mod[n^k+1, k]>0, k++ ];k)

EXAMPLE

a(6)=7 because 7 divides 6^7+1 and there doesn't exist m such that 1<m<7 and m divides 6^m+1.

MATHEMATICA

a[n_] := (For[k=2, Mod[n^k+1, k]>0, k++ ]; k); Table[a[n], {n, 100}]

CROSSREFS

Cf. A020639, A092028.

Sequence in context: A135679 A092028 A020639 * A214606 A079879 A071889

Adjacent sequences:  A092064 A092065 A092066 * A092068 A092069 A092070

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Mar 28 2004

STATUS

approved

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Last modified December 13 10:19 EST 2018. Contains 318086 sequences. (Running on oeis4.)