A090815 - OEIS

A090815

a(n)=denominator(B(2*prime(n))) where prime(n)=n-th prime and B(k) denotes the k-th Bernoulli number.

30, 42, 66, 6, 138, 6, 6, 6, 282, 354, 6, 6, 498, 6, 6, 642, 6, 6, 6, 6, 6, 6, 1002, 1074, 6, 6, 6, 6, 6, 1362, 6, 1578, 6, 6, 6, 6, 6, 6, 6, 2082, 2154, 6, 2298, 6, 6, 6, 6, 6, 6, 6, 2802, 2874, 6, 3018, 6, 6, 6, 6, 6, 3378, 6, 3522, 6, 6, 6, 6, 6, 6, 6, 6, 6, 4314, 6, 6, 6, 6, 6, 6, 6

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OFFSET

1,1

COMMENTS

If p and q=2*p+1 are both primes (Sophie Germain primes: A005384) then a(n)=6*q, otherwise a(n)=6. - Enrique Pérez Herrero, Aug 17 2011

LINKS

Enrique Pérez Herrero, Table of n, a(n) for n = 1..2000

MATHEMATICA

A090815[n_]:=If[!PrimeQ[2*Prime[n]+1], 6, 6*(2*Prime[n]+1)]; Array[A090815, 100] (* Enrique Pérez Herrero, Aug 17 2011 *)

PROG

(PARI) a(n)=denominator(bernfrac(2*prime(n)))

CROSSREFS