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%I M0396 #21 Aug 23 2023 10:39:32
%S 1,0,1,2,3,0,12,40,100,0,1225,6860,28812,0,1037232,9779616
%N Number of alternating sign n X n matrices invariant under a quarter turn.
%C Robbins incorrectly gives a(12) = 6460.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%D 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.
%H G. Kuperberg, <a href="https://arxiv.org/abs/math/0008184">Symmetry classes of alternating-sign matrices under one roof</a>, arXiv:math/0008184 [math.CO], 2000-2001.
%H D. P. Robbins, <a href="https://arxiv.org/abs/math/0008045">Symmetry classes of alternating sign matrices</a>, arXiv:math/0008045 [math.CO], 2000.
%H R. P. Stanley, <a href="/A005130/a005130.pdf">A baker's dozen of conjectures concerning plane partitions</a>, pp. 285-293 of "Combinatoire Enumerative (Montreal 1985)", Lect. Notes Math. 1234, 1986. Preprint. [Annotated scanned copy]
%F Robbins gives a simple (conjectured) formula.
%Y a(4n) gives A059476.
%K nonn,nice,more
%O 1,4
%A _N. J. A. Sloane_
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