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A099612
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Numerators of the coefficients in the Taylor expansion of sec(x) + tan(x) around x=0.
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11
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1, 1, 1, 1, 5, 2, 61, 17, 277, 62, 50521, 1382, 540553, 21844, 199360981, 929569, 3878302429, 6404582, 2404879675441, 443861162, 14814847529501, 18888466084, 69348874393137901, 113927491862, 238685140977801337, 58870668456604, 4087072509293123892361
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OFFSET
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0,5
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LINKS
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FORMULA
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Let R(x) = (-1)^floor(x/2)*(4^(x+1)-2^(x+1))*((HurwitzZeta(-x,3/4) - HurwitzZeta(-x,1/4)) /(2^(-x)-2)-Zeta(-x))/Gamma(x+1) then a(n) = numerator(R(n)) and A099617(n) = denominator(R(n)) for n>=1. - Peter Luschny, Aug 25 2015
Let F(x,t) = exp(-I*t*x)*(1+(exp(exp(I*t))-1)/(exp(2*exp(I*t))+1)) and r(x) = ((cos(x*Pi/2)+sin(x*Pi/2))/Pi)*Integral_{t=0..2*Pi} F(x,t) then a(n) = numerator(r(n)) and A099617(n) = denominator(r(n)) for n>=1. - Peter Luschny, Aug 25 2015
a(n) = numerator(2*i^(n+1)*PolyLog(-n, -i)/n!) for n>0.
a(n) = numerator(2^n*|Euler(n,1/2) - Euler(n,1)|/n!) for n>0 where Euler(n,x) are the Euler polynomials. (End)
Conjecture: For n >= 0, (-1)^n * a(n+1) is the numerator of the n-th term of the Taylor expansion of 1/(1 + sin(x)) around x = 0. [This is based on the fact that (sec(x) + tan(x))' = 1/(1 + sin(-x)). Clark Kimberling in A279107 states my conjecture as a fact, but no proof or reference is given.] - Petros Hadjicostas, Oct 06 2019
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EXAMPLE
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1 + x + 1/2*x^2 + 1/3*x^3 + 5/24*x^4 + 2/15*x^5 + 61/720*x^6 + 17/315*x^7 + ...
1, 1, 1/2, 1/3, 5/24, 2/15, 61/720, 17/315, 277/8064, 62/2835, 50521/3628800, 1382/155925, 540553/95800320, ... = A099612/A099617
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MAPLE
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R := n -> (cos(n*Pi/2)+sin(n*Pi/2))*(4^(n+1)-2^(n+1))*((Zeta(0, -n, 3/4)-Zeta(0, -n, 1/4))/(2^(-n)-2)-Zeta(-n))/GAMMA(n+1):
S := proc(n, k) option remember; if k = 0 then `if`(n = 0, 1, 0) else
S(n, k - 1) + S(n - 1, n - k) fi end: A099612 := n -> numer(S(n, n)/n!):
# or evaluating polynomials at -i:
P := proc(n, x) local k, j; add(add((-1)^j*2^(-k)*binomial(k, j)*(k-2*j)^n*
x^(n-k), j=0..k), k=0..n) end: R := n -> `if`(n = 0, 1, P(n-1, -I)/ n!):
seq(numer(R(n)), n=0..26);
# or with the Euler polynomials:
ep := n -> `if`(n=0, 1, 2^n*(euler(n, 1/2)-euler(n, 1))*(-1)^iquo(n+1, 2)):
a := n -> numer(ep(n)/n!): seq(a(n), n=0..26); # (End)
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MATHEMATICA
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nn = 26; Numerator[CoefficientList[Series[Sec[x] + Tan[x], {x, 0, nn}], x]] (* T. D. Noe, Jul 24 2013 *)
Table[If[n==0, 1, 2 I^(n+1) PolyLog[-n, -I] / n!], {n, 0, 26}] // Numerator (* Peter Luschny, Aug 03 2017 *)
Table[(1 + Mod[n, 2])LerchPhi[(-1)^(n+1), n+1, 1/2]/Pi^(n+1), {n, 0, 26}] // Numerator (* Peter Luschny, Aug 03 2017 *)
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CROSSREFS
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KEYWORD
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nonn,frac
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AUTHOR
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STATUS
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approved
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