

A005160


Number of alternating sign n X n matrices invariant under a quarter turn.
(Formerly M0396)


4



1, 0, 1, 2, 3, 0, 12, 40, 100, 0, 1225, 6860, 28812, 0, 1037232, 9779616
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OFFSET

1,4


REFERENCES

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. 285293 of "Combinatoire Enumerative (Montreal 1985)", Lect. Notes Math. 1234, 1986.


LINKS

Table of n, a(n) for n=1..16.
G. Kuperberg, Symmetry classes of alternatingsign matrices under one roof, arXiv math.CO/0008184
D. P. Robbins, Symmetry classes of alternating sign matrices, arXiv:math.CO/0008045


FORMULA

Robbins gives a simple (conjectured) formula.


CROSSREFS

a(4n) gives A059476.
Sequence in context: A002099 A219341 A059740 * A085042 A096542 A009206
Adjacent sequences: A005157 A005158 A005159 * A005161 A005162 A005163


KEYWORD

nonn,nice,more


AUTHOR

N. J. A. Sloane.


EXTENSIONS

Robbins incorrectly gives a(12) = 6460.


STATUS

approved



