OFFSET
0,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..630
Wikipedia, Lattice path
Wikipedia, Self-avoiding walk
MAPLE
b:= proc(l) option remember; `if`(l[-1]=0, 1, (r-> add(
add(add(`if`(i+j+k=1, (h-> `if`(h[1]<0, 0, b(h)))(
sort(l-[i, j, k])), 0), k=r), j=r), i=r))([$-1..1]))
end:
a:= n-> add(add(b(sort([i, j, n-i-j])), j=0..n-i), i=0..n):
seq(a(n), n=0..23);
MATHEMATICA
b[l_] := b[l] = If[Last[l] == 0, 1, Function[r, Sum[If[i + j + k == 1, Function[h, If[h[[1]] < 0, 0, b[h]]][Sort[l - {i, j, k}]], 0], {i, r}, {j, r}, {k, r}]][{-1, 0, 1}]];
a[n_] := Sum[Sum[b[Sort[{i, j, n - i - j}]], {j, 0, n - i}], {i, 0, n}];
a /@ Range[0, 23] (* Jean-François Alcover, May 13 2020, after Maple *)
CROSSREFS
KEYWORD
nonn,walk
AUTHOR
Alois P. Heinz, Oct 11 2019
STATUS
approved