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A220002
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Numerators of the coefficients of an asymptotic expansion in even powers of the Catalan numbers.
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8
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1, 5, 21, 715, -162877, 19840275, -7176079695, 1829885835675, -5009184735027165, 2216222559226679575, -2463196751104762933637, 1679951011110471133453965, -5519118103058048675551057049, 5373485053345792589762994345215, -12239617587594386225052760043303511
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OFFSET
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0,2
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COMMENTS
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Let N = 4*n+3 and A = sum_{k>=0} a(k)/(A123854(k)*N^(2*k)) then
C(n) ~ 8*4^n*A/(N*sqrt(N*Pi)), C(n) = (4^n/sqrt(Pi))*(Gamma(n+1/2)/ Gamma(n+2)) the Catalan numbers A000108.
The asymptotic expansion of the Catalan numbers considered here is based on the Taylor expansion of square root of the sine cardinal. This asymptotic series involves only even powers of N, making it more efficient than the asymptotic series based on Stirling's approximation to the central binomial which involves all powers (see for example: D. E. Knuth, 7.2.1.6 formula (16)). The series is discussed by Kessler and Schiff but is included as a special case in the asymptotic expansion given by J. L. Fields for quotients Gamma(x+a)/Gamma(x+b) and discussed by Y. L. Luke (p. 34-35), apparently overlooked by Kessler and Schiff.
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REFERENCES
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Donald E. Knuth, The Art of Computer Programming, Volume 4, Fascicle 4: Generating All Trees—History of Combinatorial Generation, 2006.
Y. L. Luke, The Special Functions and their Approximations, Vol. 1. Academic Press, 1969.
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LINKS
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FORMULA
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Let [x^n]T(f(x)) denote the coefficient of x^n in the Taylor expansion of f(x) then r(n) = (-1)^n*prod_{i=1..2n}(2i+1)*[x^(2*n)]T(sqrt(sin(x)/x)) is the rational coefficient of the asymptotic expansion (in N=4*n+3) and a(n) = numerator(r(n)) = r(n)*2^(3*n-bs(n)), where bs(n) is the binary sum of n (A000120).
Also a(n) = numerator([x^(2*n)]T(exp(S))) where S = sum_{k>=1}((4-E(2*k))/ (4*k)*x^(2*k)) and E(n) the Euler numbers A122045.
Also a(n) = sf(4*n+1)*2^(3*n-bs(n))*F_{2*n}(-1/4) where sf(n) is the swinging factorial A056040, bs(n) the binary sum of n and F_{n}(x) J. L. Fields' generalized Bernoulli polynomials A220412.
In terms of sequences this means
d(n+1) = -4*(2*n+1)*A161151(n)*d(n),
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EXAMPLE
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With N = 4*n+3 the first few terms of A are A = 1 + 5/(4*N^2) + 21/(32*N^4) + 715/(128*N^6) - 162877/(2048*N^8) + 19840275/(8192*N^10). With this A C(n) = round(8*4^n*A/(N*sqrt(N*Pi))) for n = 0..39 (if computed with sufficient numerical precision).
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MAPLE
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A220002 := proc(n) local s; s := n -> `if`(n > 0, s(iquo(n, 2))+n, 0);
(-1)^n*mul(4*i+2, i = 1..2*n)*2^s(iquo(n, 2))*coeff(taylor(sqrt(sin(x)/x), x, 2*n+2), x, 2*n) end: seq(A220002(n), n = 0..14);
# Second program illustrating J. L. Fields expansion of gamma quotients.
A220002 := proc(n) local recF, binSum, swing;
binSum := n -> add(i, i=convert(n, base, 2));
swing := n -> n!/iquo(n, 2)!^2;
recF := proc(n, x) option remember; `if`(n=0, 1, -2*x*add(binomial(n-1, 2*k+1)*bernoulli(2*k+2)/(2*k+2)*recF(n-2*k-2, x), k=0..n/2-1)) end: recF(2*n, -1/4)*2^(3*n-binSum(n))*swing(4*n+1) end:
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MATHEMATICA
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max = 14; CoefficientList[ Series[ Sqrt[ Sinc[x]], {x, 0, 2*max+1}], x^2][[1 ;; max+1]]*Table[ (-1)^n*Product[ (2*k+1), {k, 1, 2*n}], {n, 0, max}] // Numerator (* Jean-François Alcover, Jun 26 2013 *)
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PROG
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(Sage)
length = 15; T = taylor(sqrt(sin(x)/x), x, 0, 2*length+2)
P = mul(4*i+2 for i in (1..2*n)) << A005187(n//2)
return (-1)^n*P*T.coefficient(x, 2*n)
[A220002(n) for n in range(length)]
(Sage) # Second program illustrating the connection with the Euler numbers.
S = lambda n: sum((4-euler_number(2*k))/(4*k*x^(2*k)) for k in (1..n))
T = taylor(exp(S(2*n+1)), x, infinity, 2*n-1).coefficients()
return [t[0].numerator() for t in T][::-1]
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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