login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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 (n^x + 1) mod x = 0, and if n is odd, the smallest prime factor of n+1 (2) is a solution of (n^x + 1) mod 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).

From Charlie Neder, Jun 16 2019: (Start)

Theorem: a(n) = A020639(n+1).

Proof: If a(n) is composite (kp, say) then n^(kp) == -1 (mod p), but then n^k is also congruent to -1 (mod p) by Fermat's little theorem, contradicting the assumption that a(n) was minimal. Thus, a(n) must be prime, and using Fermat's little theorem again shows that n^p == -1 (mod p) iff n == -1 (mod p), and A020639(n+1) gives the least p such that this is the case. (End)

LINKS

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

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 A325643 A079879

Adjacent sequences:  A092064 A092065 A092066 * A092068 A092069 A092070

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Mar 28 2004

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 13 18:14 EDT 2019. Contains 327981 sequences. (Running on oeis4.)