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URL: https://oeis.org/A296839

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A296839
Expansion of e.g.f. tan(x*tan(x/2)) (even powers only).
10
0, 1, 1, 33, 437, 22205, 978873, 81005113, 7356832669, 949918117653, 142805534055905, 27120922891214801, 6016195462632487941, 1592800634594574194413, 486576430503128985793417, 171866951067212728072402665, 69025662074064538734826793453
OFFSET
0,4
LINKS
FORMULA
a(n) = (2*n)! * [x^(2*n)] tan(x*tan(x/2)).
a(n) ~ c * d^n * n^(2*n + 1/2) / exp(2*n), where d = 16/Pi^2 = 1.621138938277404343102071411355642222469740394755... is the root of the equation tan(1/sqrt(d)) = Pi*sqrt(d)/4 and c = 1.75568815831... - Vaclav Kotesovec, Dec 21 2017, updated Mar 16 2024
EXAMPLE
tan(x*tan(x/2)) = x^2/2! + x^4/4! + 33*x^6/6! + 437*x^8/8! + ...
MATHEMATICA
nmax = 16; Table[(CoefficientList[Series[Tan[x Tan[x/2]], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}]
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Dec 21 2017
STATUS
approved