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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified October 31 07:45 EDT 2020. Contains 338101 sequences. (Running on oeis4.)