%I #20 May 17 2021 04:09:52
%S 7,14,28,35,49,56,70,77,91,98,112,119,133,140,154,161,175,182,196,203,
%T 217,224,238,245,259,266,280,287,301,308,322,329,343,350,364,371,385,
%U 392,406,413,427,434,448,455,469,476,490,497,511,518,532,539,553,560
%N Numbers k such that 7 divides the numerator of B(2*k) where B(k) = the k-th Bernoulli number.
%H Amiram Eldar, <a href="/A089644/b089644.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = 7*A001651(n).
%F a(n) = 21*(n-1)-a(n-1), with n>1, a(1)=7. - _Vincenzo Librandi_, Nov 16 2010
%t Select[Range[500], Divisible[Numerator @ BernoulliB[2*#], 7] &] (* _Amiram Eldar_, May 17 2021 *)
%o (PARI) isok(n) = ! ((numerator(bernfrac(2*n))) % 7); \\ _Michel Marcus_, Dec 06 2013
%Y Cf. A001651, A027641, A027642, A073216.
%K nonn,easy
%O 1,1
%A _Benoit Cloitre_, Jan 01 2004