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%I #6 Nov 12 2018 03:25:01
%S 1,5,14,315,2634,301262,8035168,4451407563,392447922178,
%T 1028823851939030,306635655405986312,3743721825782942609558,
%U 3832881704712490758507152,215313862348697780762374273824,766493362781127793638943080523776,196222469681558506008588371784165178651,2452358136558134202145977229121349160510450
%N Number of symmetric {-1, +1} matrices of order n with nonnegative row and column sums.
%D I. Anderson, Combinatorics of Finite Sets, Oxford 1987, Ch. 3.1.
%D Torsten Sillke and Achim Flammenkamp, unpublished.
%H C. Greene and D. J. Kleitman, <a href="https://doi.org/10.1016/0097-3165(76)90079-0">Strong versions of Sperner's Theorem</a>, J. Combin. Theory, A 20 (1976) 80-88.
%e n=2: A(2) = 5 from ++/++, ++/+-, -+/++, -+/+-, +-/-+
%K nonn,nice
%O 1,2
%A Torsten Sillke (sillke(AT)Mathematik.Uni-Bielefeld.DE)
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