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A005160
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Number of alternating sign n X n matrices invariant under a quarter turn.
(Formerly M0396)
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4
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1, 0, 1, 2, 3, 0, 12, 40, 100, 0, 1225, 6860, 28812, 0, 1037232, 9779616
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OFFSET
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1,4
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COMMENTS
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Robbins incorrectly gives a(12) = 6460.
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
R. P. Stanley, A baker's dozen of conjectures concerning plane partitions, pp. 285-293 of "Combinatoire Enumerative (Montreal 1985)", Lect. Notes Math. 1234, 1986.
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LINKS
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FORMULA
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Robbins gives a simple (conjectured) formula.
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CROSSREFS
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KEYWORD
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nonn,nice,more
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AUTHOR
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STATUS
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approved
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