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A127358 a(n) = Sum_{k=0..n} binomial(n, floor(k/2))*2^(n-k). 8
1, 3, 8, 21, 54, 138, 350, 885, 2230, 5610, 14088, 35346, 88596, 221952, 555738, 1391061, 3480870, 8708610, 21783680, 54483510, 136254964, 340729788, 852000828, 2130354786, 5326563004 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Hankel transform is (-1)^n. In general, given r >= 0, the sequence given by Sum_{k=0..n} binomial(n, floor(k/2))*r^(n-k)} has Hankel transform (1-r)^n. The sequence is the image of the sequence with g.f. (1+x)/(1-2x) under the Chebyshev mapping g(x) -> (1/sqrt(1-4x^2))g(xc(x^2)), where c(x) is the g.f. of the Catalan numbers A000108.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 0..1000

FORMULA

G.f.: (1/sqrt(1 - 4x^2))(1 + x*c(x^2))/(1 - 2*x*c(x^2)).

a(n) = 2*a(n-1) + A054341(n-1). a(n) = Sum_{k=0..n} A126075(n,k). - Philippe Deléham, Mar 03 2007

a(n) = Sum_{k=0..n} A061554(n,k)*2^k. - Philippe Deléham, Dec 04 2009

a(n) is the sum of top row terms of M^n, M = an infinite square production matrix as follows:

2, 1, 0, 0, 0,...

1, 0, 1, 0, 0,...

0, 1, 0, 1, 0,...

0, 0, 1, 0, 1,...

0, 0, 0, 1, 0,...

... - Gary W. Adamson, Sep 07 2011

Conjecture: 2*n*a(n) + (-5*n-4)*a(n-1) + 2*(-4*n+13)*a(n-2) + 20*(n-2)*a(n-3) = 0. - R. J. Mathar, Nov 30 2012

a(n) ~ 3 * 5^n / 2^(n+1). - Vaclav Kotesovec, Feb 13 2014

EXAMPLE

a(3) = 21 = (12 + 6 + 2 + 1), where the top row of M^3 = (12, 6, 2, 1).

MATHEMATICA

Table[Sum[Binomial[n, Floor[k/2]]2^(n-k), {k, 0, n}], {n, 0, 30}] (* Harvey P. Dale, Jun 03 2012 *)

CoefficientList[Series[(1 + 2*x - Sqrt[1 - 4*x^2])/(2*Sqrt[1 - 4*x^2]*(x - 1 + Sqrt[1 - 4*x^2])), {x, 0, 50}], x] (* G. C. Greubel, May 22 2017 *)

PROG

(PARI) x='x+O('x^50); Vec((1 + 2*x - sqrt(1 - 4*x^2))/(2*sqrt(1 - 4*x^2)*(x - 1 + sqrt(1 - 4*x^2)))) \\ G. C. Greubel, May 22 2017

CROSSREFS

Cf. A107430. - Philippe Deléham, Sep 16 2009

Sequence in context: A103446 A218482 A094723 * A077849 A135473 A242452

Adjacent sequences:  A127355 A127356 A127357 * A127359 A127360 A127361

KEYWORD

easy,nonn

AUTHOR

Paul Barry, Jan 11 2007

STATUS

approved

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Last modified November 17 20:50 EST 2018. Contains 317278 sequences. (Running on oeis4.)