A107745 Smallest prime factor of 6*n-1.

5, 11, 17, 23, 29, 5, 41, 47, 53, 59, 5, 71, 7, 83, 89, 5, 101, 107, 113, 7, 5, 131, 137, 11, 149, 5, 7, 167, 173, 179, 5, 191, 197, 7, 11, 5, 13, 227, 233, 239, 5, 251, 257, 263, 269, 5, 281, 7, 293, 13, 5, 311, 317, 17, 7, 5, 11, 347, 353, 359, 5, 7, 13, 383, 389, 5, 401, 11

Offset

1,1

Comments

If 6*n-1 is prime, a(n) = 6*n-1.

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Formula

a(n) = A020639(6n-1). - R. J. Mathar, Jan 23 2007

5 <= a(n) <= 6n - 1; both bounds are sharp. - Charles R Greathouse IV, Sep 02 2024

Mathematica

a[n_]:= FactorInteger[6*n - 1][[1, 1]]; Array[a, 100]

Prog

(PARI) vector(100, n, vecmin(factor(6*n-1)[, 1])) \\ Altug Alkan, Oct 23 2015
(Magma) [Min(PrimeFactors(6*n-1)):n in[1..68]]; // Jason Kimberley, Oct 28 2015

Crossrefs

Cf. A020639, A016969, A111863.

Cf. A107744 (smallest prime factor of 6*n+1), A107746 (values of k such that A107744(k) > A107745(k)), A107747 (values of k such that A107744(k) < A107745(k)).

Sequence in context: A314240 A314241 A314242 * A111863 A333656 A043389

Adjacent sequences: A107742 A107743 A107744 * A107746 A107747 A107748

Keyword

nonn,easy,changed

Author

Zak Seidov, May 23 2005

Extensions

Comments corrected and (at the suggestion of Michel Marcus) moved to Crossrefs by Jason Kimberley, Oct 23 2015

Status

approved