A171011 Number of ways to place two non-attacking rooks on all free n-ominoes.

0, 0, 1, 9, 48, 242, 1178, 5793, 27651, 131383

Offset

1,4

Links

Table of n, a(n) for n=1..10.

Example

The triomino XX admits 1 combination of 2 non-attacking rooks.
............ X

Crossrefs

Sequence in context: A054460 A115053 A181959 * A264273 A226751 A073584

Adjacent sequences: A171008 A171009 A171010 * A171012 A171013 A171014

Keyword

nonn

Author

David Scambler, Aug 27 2010

Status

approved