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A187870 Numerator of the coefficient of x^(2n) in expansion of 1/x^4 -1/(3*x^2) -1/(x^3*arctanh(x)). 3
4, 44, 428, 10196, 10719068, 25865068, 5472607916, 74185965772, 264698472181028, 2290048394728148, 19435959308462817284, 2753151578548809148, 20586893910854623222436, 134344844535611780572028924 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Let f(x) = 1/x^4-1/(3*x^2)-1/(x^3*arctanh(x)) = sum(r(n)*x^(2n), n=0..infinity), then a(n) is the numerator of r(n), and r(n) is also the moment of order n for the density rho(x)=2*sqrt(x)/(4*(arctanh(sqrt(x)))^2+Pi^2) over the interval 0..1.

r(n) can also be evaluated as (-1)^(n+1)*det(An) with An the square matrix of order n+2 defined by: if j<=i A[i,j]=1/(2*i-2*j+3), A[i,i+1]=1, if j>i+1 A[i,j]=0.

A very similar sequence of numerators 1, 1, 4, 44, 428, 10196,... (from there on apparently the same as here) is constructed from the fractions c(0)=-1 and c(n) = sum_{i=0..n-1} c(i)/(2n-2i+1), which is c(0)=-1, c(1)=1/3, c(2)=4/45, c(3)= 44/945 etc. The recurrence is designed to ensure that sum_{i=0..n} c(i)/(2n-2i+1)=0. - Paul Curtz, Sep 15 2011

Prepending 1 to the data gives the (-1)^n times the numerator of the odd powers in the expansion of 1/arctan(x). - Peter Luschny, Oct 04 2014

LINKS

Table of n, a(n) for n=0..13.

MAPLE

A187870 := proc(n)

        1/x^4 -1/(3*x^2) -1/(x^3*arctanh(x)) ;

        coeftayl(%, x=0, 2*n) ;

        numer(%) ;

end proc:

seq(A187870(n), n=0..10) ; # R. J. Mathar, Sep 21 2011

# Or

seq((-1)^n*numer(coeff(series(1/arctan(x), x, 2*n+2), x, 2*n+1)), n=1..14); # Peter Luschny, Oct 04 2014

MATHEMATICA

a[n_] := Sum[(2^(j+1)*Binomial[2*n+3, j]*Sum[(k!*StirlingS1[j+k, j]*StirlingS2[j+1, k])/(j+k)!, {k, 0, j+1}])/(j+1), {j, 0, 2*n+3}]/ (2*n+3); Table[a[n] // Numerator, {n, 0, 13}] (* Jean-Fran├žois Alcover, Jul 03 2013, after Vladimir Kruchinin's formula in A216272 *)

CROSSREFS

Cf. A195466 (denominator).

Sequence in context: A198962 A002754 * A216272 A221405 A105038 A002278

Adjacent sequences:  A187867 A187868 A187869 * A187871 A187872 A187873

KEYWORD

nonn,frac

AUTHOR

Groux Roland, Mar 14 2011

STATUS

approved

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Last modified May 24 09:32 EDT 2017. Contains 286963 sequences.