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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 (list; graph; refs; listen; history; text; internal format)
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^(n-1)] + 1, PrimeQ]; a[0] = 1; Table[a[n], {n, 0, 20}] (* Jean-François 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 Jean-François Alcover, Dec 07 2012

STATUS

approved

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Last modified November 24 19:32 EST 2014. Contains 249909 sequences.