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A064538
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a(n) is the smallest positive integer such that a(n)*(1^n + 2^n + ... + x^n) is a polynomial in x with integer coefficients.
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15
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1, 2, 6, 4, 30, 12, 42, 24, 90, 20, 66, 24, 2730, 420, 90, 48, 510, 180, 3990, 840, 6930, 660, 690, 720, 13650, 1092, 378, 56, 870, 60, 14322, 7392, 117810, 7140, 210, 72, 1919190, 103740, 8190, 1680, 94710, 13860, 99330, 9240, 217350, 9660, 9870, 10080, 324870
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OFFSET
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0,2
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COMMENTS
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Let P_n(x) = 1^n + 2^n + ... + x^n = Sum_{i=1..n+1}c_i*x^i. Let P^*_n(x) = Sum_{i=1..n+1}(c_i/(i+1))*(x^(i+1)-x). Then b(n) = (n+1)*a(n+1)is the smallest positive integer such that b(n)*P^*_n(x) is a polynomial with integer coefficients. Proof follows from the recursion P_(n+1)(x) = x + (n+1)*P^*_n(x). As a corollary, note that, if p is the maximal prime divisor of a(n), then p<=n+1. - Vladimir Shevelev, Dec 21 2011
The recursion P_(n+1)(x) = x + (n+1)*P^*_n(x) is due to Abramovich (1973); see also Shevelev (2007). - Jonathan Sondow, Nov 16 2015
The sum S_m(n) = Sum_{k=0..n} k^m can be written as S_m(n) = n(n+1)(2n+1)P_m(n)/a(m) for even m>1, or S_m(n) = n^2*(n+1)^2*P_m(n)/a(m) for odd m>1, where a(m) is the LCM of the denominators of the coefficients of the polynomial P_m/a(m), i.e., the smallest integer such that P_m defined in this way has integer coefficients. (Cf. Michon link.) - M. F. Hasler, Mar 10 2013
a(n)/(n+1) is squarefree, by Faulhaber's formula and the von Staudt-Clausen theorem on the denominators of Bernoulli numbers. - Kieren MacMillan and Jonathan Sondow, Nov 20 2015
a(n) equals n+1 times the product of the primes p <= (n+2)/(2+(n mod 2)) such that the sum of the base-p digits of n+1 is at least p. - Bernd C. Kellner and Jonathan Sondow, May 24 2017
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REFERENCES
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M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprints), p. 804, Eq. 23.1.4.
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LINKS
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M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
Eric Weisstein's World of Mathematics, Power Sum
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FORMULA
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EXAMPLE
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1^3 + 2^3 + ... + x^3 = (x(x+1))^2/4 so a(3)=4.
1^4 + 2^4 + ... + x^4 = x(x+1)(2x+1)(3x^2+3x-1)/30, therefore a(4)=30.
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MAPLE
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# Formula of Kellner and Sondow (2017):
a := proc(n) local s; s := (p, n) -> add(i, i=convert(n, base, p));
select(isprime, [$2..(n+2)/(2+irem(n, 2))]);
(n+1)*mul(i, i=select(p->s(p, n+1)>=p, %)) end: seq(a(n), n=0..48); # Peter Luschny, May 14 2017
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MATHEMATICA
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A064538[n_] := Denominator[ Together[ (BernoulliB[n+1, x] - BernoulliB[n+1])/(n+1)]];
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PROG
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(PARI) a(n) = {my(vp = Vec(bernpol(n+1, x)-bernfrac(n+1))/(n+1)); lcm(vector(#vp, k, denominator(vp[k]))); } \\ Michel Marcus, Feb 07 2016
(Sage)
A064538 = lambda n: (n+1)*mul([p for p in (2..(n+2)//(2+n%2)) if is_prime(p) and sum((n+1).digits(base=p)) >= p])
(Python)
from __future__ import division
from sympy.ntheory.factor_ import digits, nextprime
p, m = 2, n+1
while p <= (n+2)//(2+ (n% 2)):
if sum(d for d in digits(n+1, p)[1:]) >= p:
m *= p
p = nextprime(p)
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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