%I #40 Sep 08 2022 08:45:50
%S 1,4,16,62,239,920,3544,13672,52834,204528,793092,3080226,11980667,
%T 46662704,181971248,710454896,2776717742,10863073784,42537035408,
%U 166704021596,653827252022,2566222449104,10079023179536,39611016586832
%N Expansion of (2/(3*sqrt(1-4*z)-1+4*z))*((1-sqrt(1-4*z))/(2*z))^k with k=3.
%C This sequence is the third diagonal below the main diagonal (which itself is A026641) in the array which grows with "Pascal rule" given here by rows:
%C 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,
%C 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
%C 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7,
%C 1, 2, 4, 6, 9, 12, 16, 20, 25, 30,
%C 1, 3, 7, 13, 22, 34, 50, 70, 95.
%C The Maple programs give the first diagonals of this array.
%C Apparently the number of peaks in all Dyck paths of semilength n+3 that are 1 step higher than the preceding peak. - _David Scambler_, Apr 22 2013
%H Vincenzo Librandi, <a href="/A172025/b172025.txt">Table of n, a(n) for n = 0..1000</a>
%F G.f.: (2/(3*sqrt(1-4*x)-1+4*x))*((1-sqrt(1-4*x))/(2*x))^k with k=3.
%F a(n) = Sum_{p=0..n} (-1)^(p)*binomial(2*n+k-p,n-p), with k=3.
%F a(n) ~ 2^(2*n+4)/(3*sqrt(Pi*n)). - _Vaclav Kotesovec_, Apr 19 2014
%F Conjecture: 2*n*(n+3)*a(n) + (-7*n^2 - 17*n - 8)*a(n-1) -2*(n+2)*(2*n+1)*a(n-2) = 0. - _R. J. Mathar_, Feb 19 2016
%F a(n) = [x^n] 1/((1 - x^2)*(1 - x)^(n+3)). - _Ilya Gutkovskiy_, Oct 25 2017
%e a(4) = C(11,4) - C(10,3) + C(9,2) - C(8,1) + C(7,0) = 330 - 120 + 36 - 8 + 1 = 239.
%p a:= n-> add((-1)^(p)*binomial(2*n+3-p,n-p), p=0..n):
%p seq(a(n), n=0..30);
%p # second Maple program:
%p gf:= (2/(3*sqrt(1-4*z)-1+4*z))*((1-sqrt(1-4*z))/(2*z))^3:
%p a:= n-> coeff(series(gf,z,n+10),z,n):
%p seq(a(n), n=0..30);
%t a[n_] := Binomial[2*n+3, n+3]*Hypergeometric2F1[1, -n, -3-2*n, -1]; Table[a[n], {n, 0, 23}] (* _Jean-François Alcover_, Dec 17 2013 *)
%o (PARI) k=3; my(x='x+O('x^30)); Vec((2/(3*sqrt(1-4*x)-1+4*x))*((1-sqrt(1-4*x))/(2*x))^k) \\ _G. C. Greubel_, Feb 16 2019
%o (Magma) k:=3; m:=30; R<x>:=PowerSeriesRing(Rationals(), m); Coefficients(R!( (2/(3*Sqrt(1-4*x)-1+4*x))*((1-Sqrt(1-4*x))/(2*x))^k )); // _G. C. Greubel_, Feb 16 2019
%o (Sage) k=3; ((2/(3*sqrt(1-4*x)-1+4*x))*((1-sqrt(1-4*x))/(2*x))^k).series(x, 20).coefficients(x, sparse=False) # _G. C. Greubel_, Feb 16 2019
%Y Cf. A091526 (k=-2), A072547 (k=-1), A026641 (k=0), A014300 (k=1), A014301 (k=2), A172061 (k=4), A172062 (k=5), A172063 (k=6), A172064 (k=7), A172065 (k=8), A172066 (k=9), A172067 (k=10).
%K easy,nonn
%O 0,2
%A _Richard Choulet_, Jan 23 2010