login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A125255 Smallest prime divisor of 4n-1. 2
3, 7, 11, 3, 19, 23, 3, 31, 5, 3, 43, 47, 3, 5, 59, 3, 67, 71, 3, 79, 83, 3, 7, 5, 3, 103, 107, 3, 5, 7, 3, 127, 131, 3, 139, 11, 3, 151, 5, 3, 163, 167, 3, 5, 179, 3, 11, 191, 3, 199, 7, 3, 211, 5, 3, 223, 227, 3, 5, 239, 3, 13, 251, 3, 7, 263, 3, 271, 5, 3, 283, 7, 3, 5, 13, 3, 307 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All divisors of 4n-1 are odd.

LINKS

N. Hobson, Table of n, a(n) for n = 1..1000

N. Hobson, Home page (listed in lieu of email address)

FORMULA

a(3k+1) = 3, k = 0,1,2,... a(5k+4) = 5 if k is not a multiple of 3; k = 1,2,4,5,7,8,... - Alexander Adamchuk, Nov 28 2006

a(n) = A020639(4n-1). - R. J. Mathar, Jan 23 2007

EXAMPLE

The prime divisors of 4*9 - 1 = 35 are 5 and 7, so a(9) = 5.

PROG

(PARI) vector(77, n, factor(4*n-1)[1, 1])

CROSSREFS

Cf. A004767, A107745, A125254.

Sequence in context: A265686 A042775 A229596 * A125254 A093931 A335980

Adjacent sequences:  A125252 A125253 A125254 * A125256 A125257 A125258

KEYWORD

easy,nonn

AUTHOR

Nick Hobson, Nov 26 2006

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 January 25 06:09 EST 2021. Contains 340416 sequences. (Running on oeis4.)