

A219196


A subsequence of the Bernoulli numbers denominators: a(n) = A027642(A131577(n)).


0



1, 2, 6, 30, 30, 510, 510, 510, 510, 131070, 131070, 131070, 131070, 131070, 131070, 131070, 131070, 8589934590, 8589934590, 8589934590, 8589934590
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OFFSET

0,2


COMMENTS

Conjecture: a(15) = a(16) = 131070, a(17) through a(32) = 8589934590.
Number of different terms: 1, 1, 1, 2, 4,... = abs(A141531)?
Factorization of terms from 2:
2 = 2
6 = 2*3
30 = 2*3*5
510 = 2*3*5*17
131070 = 2*3*5*17*257
8589934590 = 2*3*5*17*257*65537.
Note that all shown factors are Fermat numbers (see A092506, A019434, A000215).


LINKS

Table of n, a(n) for n=0..20.


MATHEMATICA

a[n_] := a[n] = Times @@ Select[ Divisors[2^(n1)] + 1, PrimeQ]; a[0] = 1; Table[a[n], {n, 0, 20}] (* JeanFrançois Alcover, Dec 07 2012 *)


PROG

(PARI) a(n) = denominator(bernfrac(1<<n)); \\ Michel Marcus, Aug 14 2013


CROSSREFS

Sequence in context: A095198 A126989 A128040 * A233358 A241557 A006954
Adjacent sequences: A219193 A219194 A219195 * A219197 A219198 A219199


KEYWORD

nonn


AUTHOR

Paul Curtz, Nov 14 2012


EXTENSIONS

Extended up to a(20) by JeanFrançois Alcover, Dec 07 2012


STATUS

approved



