A326727 -id:A326727 - cp's OEIS frontend

A332300 The least prime factor of the numerator of Bernoulli(2*n), or 1 if the numerator is 1.

1, 1, 1, 1, 1, 5, 691, 7, 3617, 43867, 283, 11, 103, 13, 7, 5, 37, 17, 26315271553053477373, 19, 137616929, 1520097643918070802691, 11, 23, 653, 5, 13, 39409, 7, 29, 2003, 31, 1226592271, 11, 17, 5, 3112655297839, 37, 19, 13, 631, 41, 233, 43, 11, 5, 23, 47, 7823741903

Offset: 0

Amiram Eldar, Feb 09 2020

nonn

a(n)=5 if and only if n is in A017329. - Robert Israel, Feb 09 2020
From Chai Wah Wu, Feb 10 2020: (Start)
For n > 1, clearly if a(n) = n, then n is prime. However, the converse is not true. Prime numbers p such that a(p) != p are: 2, 3, 109, 167, 211, 227, 271, ...
Conjecture: for prime p > 3, p is a prime factor of the numerator of Bernoulli(2*p), thus the conjecture implies that a(p) <= p for prime p.
(End)

			a(10) = 283, since Bernoulli(2*10) = -174611/330, and 283 is the least prime factor of its numerator, 174611 = 283 * 617.

Chai Wah Wu, Table of n, a(n) for n = 0..191 (n = 0..103 from Amiram Eldar)
S. S. Wagstaff, Jr., Factors of Bernoulli numbers.

Cf. A000367, A017329, A020639, A079294, A090947, A242193, A326727.

[n le 4 select 1 else Min(PrimeDivisors(Abs(Numerator(Bernoulli(2*n))))):n in [0..48]]; // Marius A. Burtea, Feb 09 2020

Array[FactorInteger[Abs @ Numerator @  BernoulliB[2*#]][[1, 1]] &, 30, 0]

a(n) = my(x=abs(numerator(bernfrac(2*n)))); if (x==1, 1, vecmin(factor(x)[,1])); \\ Michel Marcus, Feb 09 2020

from sympy import bernoulli, primefactors
def A332300(n):
    x = abs(bernoulli(2*n).p)
    return 1 if x == 1 else min(primefactors(x)) # Chai Wah Wu, Feb 10 2020

a(n) = A020639(abs(A000367(n))).

A326726 The prime factorization of abs(E(2k)) for k >= 2, E(k) the k-th Euler number. Factors sorted by size with the smallest factor negated. a(n) = -1 by convention for n = 1, 2.

Original entry on oeis.org

-1, -1, -5, -61, -5, 277, -19, 2659, -5, 13, 43, 967, -47, 4241723, -5, 17, 228135437, -79, 349, 87224971, -5, 5, 41737, 354957173, -31, 1567103, 1427513357, -5, 13, 2137, 111691689741601, -67, 61001082228255580483, -5, 19, 29, 71, 30211, 2717447, 77980901

Offset: 1

Peter Luschny, Jul 29 2019

For small Euler numbers the factorizations were computed with SageMath, see the b-file for the script. For larger Euler numbers the values were taken from the table of S. S. Wagstaff, Jr..
The smallest factor was negated only to be able to distinguish the individual factorizations easily. (No general formula for the number of factors is known.)
The factorizations listed in the b-file currently go up to E(164) (the prime factors of E(166) are not yet known).

			The data is given as a flatted list of factorizations written with the conventions
stated above. Because it is a list the offset is 1. The list starts:
[[-1], [-1], [-5], [-61], [-5, 277], [-19, 2659], [-5, 13, 43, 967], [-47, 4241723], [-5, 17, 228135437], [-79, 349, 87224971], [-5, 5, 41737, 354957173], ... ].
The first few factorizations are:
E(4)  = 5;
E(6)  = 61;
E(8)  = 5 * 277;
E(10) = 19 * 2659;
E(12) = 5 * 13 * 43 * 967;
E(14) = 47 * 4241723;
E(16) = 5 * 17 * 228135437;
E(18) = 79 * 349 * 87224971;
E(20) = 5 * 5 * 41737 * 354957173;
E(22) = 31 * 1567103 * 1427513357;
E(24) = 5 * 13 * 2137 * 111691689741601;

Peter Luschny, Table of n, a(n) for n = 1..428
factordb, Status of E(166).
S. S. Wagstaff, Prime factors of the absolute values of Euler numbers

Cf. A122045, A000364, A326727.

Sage
```
# See b-file.
```

cp's OEIS Frontend

A332300 The least prime factor of the numerator of Bernoulli(2*n), or 1 if the numerator is 1.

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