OFFSET
0,3
COMMENTS
a(n) is also the number of ways to place n nonattacking pieces rook + leaper[1,3] on an n X n chessboard (in fairy chess the leaper[1,3] is called a camel).
LINKS
V. Kotesovec, Number of ways of placing non-attacking queens, kings, bishops and knights (in English and Czech)
FORMULA
Asymptotic: a(n)/n! ~ 1/e^4.
MATHEMATICA
A189358[n_] := Module[{p, c = 0, i = 1, q},
p=Permutations[Range[n]]; While[i <= Length[p], q = p[[i]]; i++;
If[AllTrue[Range[n - 1], Abs[q[[# + 1]] - q[[#]]] != 3 &] &&
AllTrue[Range[n - 3], Abs[q[[# + 3]] - q[[#]]] != 1 &], c++]]; c];
Table[A189358[n], {n, 0, 9}] (* Robert Price, Apr 04 2019 *)
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Vaclav Kotesovec, Apr 23 2011
EXTENSIONS
a(17) from Alois P. Heinz, Mar 19 2017
STATUS
approved