A189566 Number of permutations p of 1,2,...,n satisfying |p(i+2)-p(i)|<>4 and |p(j+4)-p(j)|<>2 for all i=1..n-2, j=1..n-4.

1, 2, 6, 24, 60, 208, 1184, 7840, 51636, 410272, 3836456, 39971896, 455888312, 5717233896, 78164908748, 1153568477544

Offset

1,2

Comments

a(n) is also the number of ways to place n nonattacking pieces rook + leaper[2,4] on an n X n chessboard.

Links

Table of n, a(n) for n=1..16.

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.

Crossrefs

Cf. A137774, A189358, A189563, A189564, A189565.

Sequence in context: A364424 A022917 A189855 * A178009 A357249 A107762

Adjacent sequences: A189563 A189564 A189565 * A189567 A189568 A189569

Keyword

nonn,hard

Author

Vaclav Kotesovec, Apr 23 2011

Status

approved