A125255 Smallest prime divisor of 4n-1.

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

Offset

1,1

Comments

All divisors of 4n-1 are odd.

Links

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

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.

Mathematica

Table[FactorInteger[4n-1][[1, 1]], {n, 77}] (* James C. McMahon, Dec 16 2024 *)

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