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 A090987 a(n) is the smallest prime whose square divides the numerator of the Bernoulli number B(A090997(n)). 5

%I

%S 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,

%T 7,13,5,23,7

%N a(n) is the smallest prime whose square divides the numerator of the Bernoulli number B(A090997(n)).

%C 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

%H The Bernoulli Number Page, <a href="https://www.bernoulli.org/download/bn_factors.txt">Table of factors of the numerators of Bernoulli numbers Bn in the range n = 2..10000</a>, 2018.

%H S. S. Wagstaff, Jr, <a href="http://www.cerias.purdue.edu/homes/ssw/bernoulli/bnum">Prime factors of the absolute values of Bernoulli numerators</a>, 2018.

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

%K more,nonn

%O 1,1

%A _N. J. A. Sloane_, Feb 28 2004

%E 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

%E More terms from _Alexander Adamchuk_, Aug 19 2006

%E Various sections edited by _Petros Hadjicostas_, May 12 2020

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Last modified September 18 22:25 EDT 2020. Contains 337174 sequences. (Running on oeis4.)