OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..120
FORMULA
a(n) == 1 (mod 6).
MAPLE
b:= proc(n, l) option remember; `if`(n=0, 1, (k-> `if`(n>min(l),
add(`if`(l[i]=0, 0, b(n-1, sort(subsop(i=l[i]-1, l)))),
i=1..k)+b(n-1, map(x-> x+1, l)), (k+1)^n))(nops(l)))
end:
a:= n-> b(n, [0$6]):
seq(a(n), n=0..27);
CROSSREFS
KEYWORD
nonn,walk
AUTHOR
Alois P. Heinz, Jul 11 2021
STATUS
approved