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A171011
Number of ways to place two non-attacking rooks on all free n-ominoes.
0
0, 0, 1, 9, 48, 242, 1178, 5793, 27651, 131383
OFFSET
1,4
EXAMPLE
The triomino XX admits 1 combination of 2 non-attacking rooks.
............ X
CROSSREFS
Sequence in context: A054460 A115053 A181959 * A264273 A226751 A073584
KEYWORD
nonn
AUTHOR
David Scambler, Aug 27 2010
STATUS
approved