OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..300
A. Bostan and M. Kauers, 2008. Automatic Classification of Restricted Lattice Walks, ArXiv 0811.2899.
FORMULA
a(n) == 1 (mod 3). - Alois P. Heinz, Jul 12 2021
MAPLE
b:= proc(n, l) option remember; `if`(n=0, 1, b(n-1, map(x-> x+1, l))+
add(`if`(l[i]>0, b(n-1, sort(subsop(i=l[i]-1, l))), 0), i=1..3))
end:
a:= n-> b(n, [0$3]):
seq(a(n), n=0..30); # Alois P. Heinz, Jan 26 2021
MATHEMATICA
aux[i_Integer, j_Integer, k_Integer, n_Integer] := Which[Min[i, j, k, n] < 0 || Max[i, j, k] > n, 0, n == 0, KroneckerDelta[i, j, k, n], True, aux[i, j, k, n] = aux[-1 + i, -1 + j, -1 + k, -1 + n] + aux[i, j, 1 + k, -1 + n] + aux[i, 1 + j, k, -1 + n] + aux[1 + i, j, k, -1 + n]]; Table[Sum[aux[i, j, k, n], {i, 0, n}, {j, 0, n}, {k, 0, n}], {n, 0, 10}]
CROSSREFS
KEYWORD
nonn,walk
AUTHOR
Manuel Kauers, Nov 18 2008
STATUS
approved