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A185424
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Numerators of generalized Bernoulli numbers associated with the zigzag numbers A000111.
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6
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1, -1, 1, -1, 19, -5, 253, -61, 3319, -1385, 222557, -50521, 422152729, -2702765, 59833795, -199360981, 439264083023, -19391512145, 76632373664299, -2404879675441, 4432283799315809, -370371188237525
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OFFSET
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0,5
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COMMENTS
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DEFINITION
Let E(t) = sec(t)+tan(t) denote the generating function for the zigzag numbers A000111. The zigzag Bernoulli numbers, denoted ZB(n), are defined by means of the generating function
(1)... log E(t)/(E(t)-1) = sum {n = 0..inf} ZB(n)*t^n/n!.
Notice that if we were to take E(t) equal to exp(t) then (1) would be the defining function for the classical Bernoulli numbers B_n. The first few even-indexed values of ZB(n) are
....n..|..0...2.....4.......6........8..........10...........12....
===================================================================
.ZB(n).|..1..1/6..19/30..253/42..3319/30..222557/66..422152729/2730
while the odd-indexed values begin
....n..|. ..1......3......5.......7........9.........11..
=========================================================
.ZB(n).|. -1/2...-1/2...-5/2...-61/2...-1385/2...-50521/2
The present sequence gives the numerators of the zigzag Bernoulli numbers. It is not difficult to show that the odd-indexed value ZB(2*n+1) equals -1/2*A000364(n). The numerators of the even-indexed values ZB(2*n) are shown separately in A185425.
VON STAUDT-CLAUSEN THEOREM
The following analog of the von Staudt-Clausen theorem holds:
(2)... ZB(2*n) + 1/2 + S(1) + (-1)^(n+1)*S(3) equals an integer, where
... S(1) = sum {prime p, p = 1 (mod 4), p-1|2*n} 1/p,
... S(3) = sum {prime p, p = 3 (mod 4), p-1|2*n} 1/p.
For example,
(3)... ZB(12) + 1/2 + (1/5+1/13) - (1/3+1/7) = 154635.
Further examples are given below.
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LINKS
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Table of n, a(n) for n=0..21.
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FORMULA
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SEQUENCE ENTRIES
a(n) = numerator of the rational number ZB(n) where
(1)... ZB(n) = (-1)^(n*(n-1)/2)*sum {k = 0..n} binomial(n,k)/(k+1)* Bernoulli(n- k)*Euler(k).
For odd indices this simplifies to
(2)... ZB(2*n+1) = (-1)^n*Euler(2*n)/2, where Euler(2*n) = A028296(n).
For even indices we have
(3)... ZB(2*n) = (-1)^n*sum {k = 0..n} binomial(2*n,2*k)/(2*k+1)* Bernoulli(2*n- 2*k)*Euler(2*k).
GENERATING FUNCTION
E.g.f:
(4)... log(sec(t)+tan(t))/(sec(t)+tan(t)-1) =
1 -1/2*t +1/6*t^2/2! -1/2*t^3/3! + ....
RELATION WITH ZIGZAG POLYNOMIALS OF A147309
The classical Bernoulli numbers B_n are given by the double sum
(5)... B_n = sum {k=0..n} sum {j=0..k} (-1)^j*binomial(k,j)*j^n/(k+1).
The corresponding formula for the zigzag Bernoulli numbers is
(6)... ZB(n) = sum {k=0..n} sum {j=0..k}(-1)^j*binomial(k,j)*Z(n,j)/(k+1), where Z(n,x) is a zigzag polynomial as defined in A147309. Umbrally, we can express this as
(7)... ZB(n) = Z(n,B), where on the lhs the understanding is that in the expansion of the zigzag polynomial Z(n,x) a term such as c_k*x^k is to be replaced with c_k*B_k. For example, Z(6,x) = 40*x^2+20*x^4+x^6 and so ZB(6) = 40*B_2+20*B_4+B_6 = 40*(1/6)+20*(-1/30)+(1/42) = 253/42.
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EXAMPLE
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Examples of von Staudt and Clausen's theorem for ZB(2*n):
ZB(2) = 1/6 = 1 - 1/2 - 1/3;
ZB(4) = 19/30 = 1 - 1/2 + 1/3 - 1/5;
ZB(6) = 253/42 = 7 - 1/2 - 1/3 - 1/7;
ZB(8) = 3319/30 = 111 - 1/2 + 1/3 - 1/5;
ZB(10) = 222557/66 = 3373 - 1/2 - 1/3 - 1/11.
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MAPLE
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#A185424
a:= n-> numer((-1)^(n*(n-1)/2)*add(binomial(n, k)/(k+1)* bernoulli(n-k) *euler(k), k = 0..n)):
seq(a(n), n = 0..20);
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MATHEMATICA
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Numerator[ Range[0, 30]! CoefficientList[ Series[Log(Sec[x]+Tan[x])/(Sec[x] +Tan[x] - 1), {x, 0, 30}], x]]
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CROSSREFS
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Cf. A000111, A027641, A027642, A147309, A185425.
Sequence of denominators is A141056.
Sequence in context: A089572 A040348 A128050 * A040347 A040346 A124608
Adjacent sequences: A185421 A185422 A185423 * A185425 A185426 A185427
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KEYWORD
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easy,sign,frac
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AUTHOR
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Peter Bala, Feb 18 2011
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STATUS
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approved
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