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A138903 a(n) = (1/2^n)* Sum_{k=0..n} binomial(n,k)*(n+k)^(n-1). 2
1, 3, 21, 234, 3590, 70254, 1672972, 46955760, 1517994792, 55549351800, 2269918543640, 102452561694864, 5062050729973120, 271751784988056576, 15750949414628405760, 980315266648197537792, 65207656047198387921536 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..17.

FORMULA

E.g.f.: A(x) = log(B(x)), where B(x) is e.g.f. of A138860.

E.g.f: A(x) = Series_Reversion[ 2*x/(exp(x) + exp(2*x)) ].

a(n) ~ n^(n-1)*(1+r)^n*r^n/(sqrt(1+3*r)*(1-r)^(2*n)*exp(n)*2^n), where r = 0.6472709258412625... is the root of the equation (r/(1-r))^(1+r) = e. - Vaclav Kotesovec, Jun 15 2013

MAPLE

A138903 := proc(n) local k ; add(binomial(n, k)*(n+k)^(n-1), k=0..n)/2^n ; end: seq(A138903(n), n=1..20) ; # R. J. Mathar, Apr 12 2008

MATHEMATICA

Table[1/2^n * Sum[Binomial[n, k]*(n+k)^(n-1), {k, 0, n}], {n, 1, 20}] (* Vaclav Kotesovec, Jun 15 2013 *)

PROG

(PARI) {a(n)=local(X=x+x*O(x^n)); n!*polcoeff(serreverse(2*x/(exp(X)+exp(2*X)) ), n)}

CROSSREFS

Sequence in context: A005373 A078586 A179331 * A302703 A334262 A234855

Adjacent sequences:  A138900 A138901 A138902 * A138904 A138905 A138906

KEYWORD

easy,nonn

AUTHOR

Paul Hanna and Vladeta Jovovic, Apr 02 2008, Apr 03 2008

EXTENSIONS

More terms from R. J. Mathar, Apr 12 2008

STATUS

approved

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Last modified May 29 03:31 EDT 2022. Contains 354122 sequences. (Running on oeis4.)