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A009011
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Expansion of e.g.f.: 1/cos(tanh(x)) (even-indexed coefficients only).
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7
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1, 1, -3, -3, 665, -25031, 607893, 13065717, -3684215119, 322746228337, -2173907680851, -5317484317809939, 1007358319257596489, 46704901267812186793, -111828938184578058229947, 54497429277597236190513381, -19349766542166780916394521759
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OFFSET
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0,3
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LINKS
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EXAMPLE
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1/cos(tanh x) = 1+ 1*x^2/2! -3*x^4/4! -3*x^6/6! +665*x^8/8! -25031*x^10/10! +... = 1 +x^2/2 -x^4/8 -x^6/240 +19*x^8/1152 -25031*x^10/3628800 +...
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MATHEMATICA
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f[x_] := Sec@Tanh[x]; Table[Derivative[2*n][f][0], {n, 0, 16}] (* Arkadiusz Wesolowski, Aug 18 2012 *)
With[{nmax = 50}, CoefficientList[Series[1/(Cos[Tanh[x]]), {x, 0, nmax}], x]*Range[0, nmax]!][[1 ;; -1 ;; 2]] (* G. C. Greubel, Jul 21 2018 *)
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PROG
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(PARI) x='x+O('x^50); v=Vec(serlaplace(1/cos(tanh(x)))); vector(#v\2, n, v[2*n-1]) \\ G. C. Greubel, Jul 21 2018
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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