OFFSET
1,2
COMMENTS
Column 4 of A204213.
Number of excursions (walks starting at the origin, ending on the x-axis, and never go below the x-axis in between) with n steps from {-4,-3,-2,-1,0,1,2,3,4}. - David Nguyen, Dec 16 2016
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
C. Banderier, C. Krattenthaler, A. Krinik, D. Kruchinin, V. Kruchinin, D. Nguyen, and M. Wallner, Explicit formulas for enumeration of lattice paths: basketball and the kernel method, arXiv preprint arXiv:1609.06473 [math.CO], 2016.
FORMULA
a(n) = Sum_{i=1..n} ((Sum_{j=0..(4*i)/9} (binomial(i,j)*binomial(-9*j+5*i-1,4*i-9*j)*(-1)^j))*a(n-i))/n. - Vladimir Kruchinin, Apr 06 2017
EXAMPLE
Some solutions for n=5
..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
..3....3....3....2....1....2....4....4....3....3....2....2....0....1....4....4
..0....2....5....1....3....0....2....2....2....5....1....0....3....5....3....6
..0....1....6....2....4....3....1....3....3....2....2....1....3....3....1....3
..3....3....3....2....3....3....3....4....2....3....0....1....3....1....2....0
..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
MATHEMATICA
a[n_] := a[n] = If[n == 0, 1, Sum[(Sum[Binomial[i, j] Binomial[-9j + 5i - 1, 4i - 9j] (-1)^j, {j, 0, (4i)/9}]) a[n - i], {i, 1, n}]/n];
a /@ Range[1, 21] (* Jean-François Alcover, Sep 24 2019, after Vladimir Kruchinin *)
PROG
(Maxima)
a(n):=if n=0 then 1 else sum((sum(binomial(i, j)*binomial(-9*j+5*i-1, 4*i-9*j)*(-1)^j, j, 0, (4*i)/9))*a(n-i), i, 1, n)/n; /* Vladimir Kruchinin, Apr 06 2017 */
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Jan 12 2012
STATUS
approved