OFFSET
0,3
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Steffen Eger, On the Number of Many-to-Many Alignments of N Sequences, arXiv:1511.00622 [math.CO], 2015.
FORMULA
G.f.: 1/sqrt(1-2*x-3*x^2-4*x^3). - Mark van Hoeij, Apr 16 2013
G.f.: Q(0), where Q(k) = 1 + x*(2+3*x+4*x^2)*(4*k+1)/( 4*k+2 - x*(2+3*x+4*x^2)*(4*k+2)*(4*k+3)/(x*(2+3*x+4*x^2)*(4*k+3) + 4*(k+1)/Q(k+1) )); (continued fraction). - Sergei N. Gladkovskii, Sep 14 2013
a(n) = Sum_{k=0..n} (binomial(2*k,k) * Sum_{j=0..k} (binomial(j,n-k-j) *binomial(k,j)*2^(j-k)*3^(-n+k+2*j)*4^(n-k-2*j))). - Vladimir Kruchinin, Feb 27 2016
D-finite with recurrence: +(n)*a(n) +(-2*n+1)*a(n-1) +3*(-n+1)*a(n-2) +2*(-2*n+3)*a(n-3)=0. - R. J. Mathar, Jan 14 2020
MATHEMATICA
a[n_]:= Sum[Binomial[2k, k]*Sum[Binomial[j, n-k-j]*Binomial[k, j]*2^(j-k) *3^(-n+k+2j)*4^(n-k-2j), {j, 0, k}], {k, 0, n}];
Array[a, 30, 0] (* Jean-François Alcover, Jul 21 2018, after Vladimir Kruchinin *)
CoefficientList[Series[1/Sqrt[1-2*x-3*x^2-4*x^3], {x, 0, 30}], x] (* G. C. Greubel, Feb 18 2019 *)
PROG
(PARI) /* same as in A092566 but use */
steps=[[1, 0], [1, 1], [1, 2], [2, 1]];
/* Joerg Arndt, Jun 30 2011 */
(PARI) my(x='x+O('x^30)); Vec(1/sqrt(1-2*x-3*x^2-4*x^3)) \\ G. C. Greubel, Feb 18 2019
(Maxima)
a(n):=sum(binomial(2*k, k) * sum(binomial(j, n-k-j) * 2^(j-k) * binomial(k, j) * 3^(-n+k+2*j) * 4^(n-k-2*j), j, 0, k), k, 0, n); /* Vladimir Kruchinin, Feb 27 2016 */
(Magma) m:=30; R<x>:=PowerSeriesRing(Rationals(), m); Coefficients(R!( 1/Sqrt(1-2*x-3*x^2-4*x^3) )); // G. C. Greubel, Feb 18 2019
(Sage) (1/sqrt(1-2*x-3*x^2-4*x^3)).series(x, 30).coefficients(x, sparse=False) # G. C. Greubel, Feb 18 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Joerg Arndt, Jun 30 2011
STATUS
approved