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A090987 a(n) is the smallest prime whose square divides the numerator of the Bernoulli number B(A090997(n)). 5
5, 7, 5, 7, 103, 11, 5, 37, 7, 13, 5, 7, 5, 11, 7, 5, 17, 5, 13, 7, 19, 11, 5, 17, 5, 59, 5, 11, 7, 13, 5, 23, 7 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

It appears that, except for irregular primes belonging to A094095, such as a(5) = 103, a(8) = 37 and a(26) = 59, all regular prime a(n) = p divide the corresponding numerators of the Bernoulli numbers B(A090997(n)) with indices of the form 2*k*p^2, where k > 0 is an integer. - Alexander Adamchuk, Aug 19 2006

LINKS

Table of n, a(n) for n=1..33.

The Bernoulli Number Page, Table of factors of the numerators of Bernoulli numbers Bn in the range n = 2..10000, 2018.

S. S. Wagstaff, Jr, Prime factors of the absolute values of Bernoulli numerators, 2018.

CROSSREFS

Cf. A000367, A090997, A094095, A122270, A122271, A122272, A122273.

Sequence in context: A010718 A247872 A306400 * A278813 A217167 A195498

Adjacent sequences:  A090984 A090985 A090986 * A090988 A090989 A090990

KEYWORD

more,nonn

AUTHOR

N. J. A. Sloane, Feb 28 2004

EXTENSIONS

In view of the phrase "it appears", it is not clear to me that the correctness of this sequence has been rigorously established. - N. J. A. Sloane, Aug 26 2006

More terms from Alexander Adamchuk, Aug 19 2006

Various sections edited by Petros Hadjicostas, May 12 2020

STATUS

approved

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Last modified August 9 22:52 EDT 2020. Contains 336335 sequences. (Running on oeis4.)