A089644 Numbers k such that 7 divides the numerator of B(2*k) where B(k) = the k-th Bernoulli number.

7, 14, 28, 35, 49, 56, 70, 77, 91, 98, 112, 119, 133, 140, 154, 161, 175, 182, 196, 203, 217, 224, 238, 245, 259, 266, 280, 287, 301, 308, 322, 329, 343, 350, 364, 371, 385, 392, 406, 413, 427, 434, 448, 455, 469, 476, 490, 497, 511, 518, 532, 539, 553, 560

Offset: 1

Author

Benoit Cloitre, Jan 01 2004

Keywords

• nonn
• easy

Links

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

Crossrefs

Cf. A001651, A027641, A027642, A073216.

Programs

Mathematica

Select[Range[500], Divisible[Numerator @ BernoulliB[2*#], 7] &] (* Amiram Eldar, May 17 2021 *)

PARI

isok(n) = ! ((numerator(bernfrac(2*n))) % 7); \\ Michel Marcus, Dec 06 2013

Formula

a(n) = 7*A001651(n).

a(n) = 21*(n-1)-a(n-1), with n>1, a(1)=7. - Vincenzo Librandi, Nov 16 2010