login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A009011 Expansion of e.g.f.: 1/cos(tanh(x)) (even-indexed coefficients only). 7
1, 1, -3, -3, 665, -25031, 607893, 13065717, -3684215119, 322746228337, -2173907680851, -5317484317809939, 1007358319257596489, 46704901267812186793, -111828938184578058229947, 54497429277597236190513381, -19349766542166780916394521759 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..200 (terms 0..50 from Vincenzo Librandi)

EXAMPLE

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 +...

MATHEMATICA

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 *)

PROG

(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

CROSSREFS

Sequence in context: A242886 A221947 A138662 * A291095 A009498 A076253

Adjacent sequences:  A009008 A009009 A009010 * A009012 A009013 A009014

KEYWORD

sign

AUTHOR

R. H. Hardin

EXTENSIONS

Extended with signs by Olivier Gérard, Mar 15 1997

a(15), a(16) from Arkadiusz Wesolowski, Aug 18 2012

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified August 23 12:02 EDT 2019. Contains 326222 sequences. (Running on oeis4.)