The On-Line Encyclopedia of Integer Sequences (OEIS)

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A116895

Least prime factor of n^n-1.
(history; published version)

#6 by Alois P. Heinz at Wed Dec 19 19:02:53 EST 2018

STATUS

editing

approved

#5 by Alois P. Heinz at Wed Dec 19 19:02:50 EST 2018

COMMENTS

If n is odd then a(n)=2; also, if n is even and not divisible by 3 then a(n)=3. - _Zak Seidov, _, Mar 03 2006

STATUS

proposed

editing

#4 by Antti Karttunen at Wed Dec 19 18:39:49 EST 2018

STATUS

editing

proposed

#3 by Antti Karttunen at Wed Dec 19 17:18:28 EST 2018

LINKS

Antti Karttunen, <a href="/A116895/b116895.txt">Table of n, a(n) for n = 2..16384</a>

PROG

(PARI) A116895(n) = { my(k=(n^n)-1); forprime(p=2, , if(!(k%p), return(p))); }; \\ Antti Karttunen, Dec 19 2018

STATUS

approved

editing

#2 by Russ Cox at Fri Mar 30 17:40:41 EDT 2012

AUTHOR

_Giovanni Resta (g.resta(AT)iit.cnr.it), _, Mar 02 2006

Discussion

Fri Mar 30

17:40

OEIS Server: https://oeis.org/edit/global/192

#1 by N. J. A. Sloane at Fri Feb 24 03:00:00 EST 2006

NAME

Least prime factor of n^n-1.

DATA

3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 13, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 7, 2

OFFSET

2,1

COMMENTS

If n is odd then a(n)=2; also, if n is even and not divisible by 3 then a(n)=3. - Zak Seidov, Mar 03 2006

EXAMPLE

6^6-1=5*7*31*43, so a(6)=5.

MATHEMATICA

Table[FactorInteger[GCD[n^n-1, 200! ]][[1, 1]], {n, 2, 130}]

CROSSREFS

Cf. A006486, A007571, A048861.

KEYWORD

nonn

AUTHOR

Giovanni Resta (g.resta(AT)iit.cnr.it), Mar 02 2006

STATUS

approved