A090818 Nonprime numbers k such that 3*(3^k-1)*B(2*k) is an integer, where B(k) denotes the k-th Bernoulli number.

1, 4, 12, 16, 25, 28, 32, 35, 36, 48, 49, 52, 55, 64, 65, 76, 77, 80, 84, 85, 91, 92, 95, 96, 108, 112, 115, 119, 121, 124, 125, 133, 143, 144, 145, 148, 155, 156, 160, 161, 164, 169, 172, 175, 185, 187, 188, 192, 196, 203, 205, 208, 209, 212, 215, 217, 221, 228

Offset

1,2

Comments

It appears that a(n) is never == 2 (mod 4).

Links

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

Mathematica

q[k_] := !PrimeQ[k] && IntegerQ[3*(3^k-1)*BernoulliB[2*k]]; Select[Range[250], q] (* Amiram Eldar, Apr 20 2025 *)

Prog

(PARI) isok(k) = !isprime(k) && denominator(3*(3^k-1)*bernfrac(2*k)) == 1; \\ Amiram Eldar, Apr 20 2025

Crossrefs

Cf. A027641, A027642.

Sequence in context: A186303 A073687 A187084 * A075191 A320922 A028594

Adjacent sequences: A090815 A090816 A090817 * A090819 A090820 A090821

Keyword

nonn

Author

Benoit Cloitre, Feb 11 2004

Status

approved