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%I #5 Mar 14 2020 18:54:09
%S 1,1,2,5,66,7937,10211144,133506398361,18551599312980440,
%T 28652629505982770906471,510824181488832447063505273252
%N Number of non-isomorphic 2n X 2n symmetric matrices with entries in {+1, -1} and all rows and columns summing to zero.
%C a(n) is the number of non-isomorphic 2n X 2n symmetric binary matrices with n ones in every row and column.
%F a(n) = A333159(2*n, n).
%e The a(1) = 1 matrix is:
%e [+ -]
%e [- +]
%e The a(2) = 2 matrices are:
%e [+ + - -] [+ + - -]
%e [+ + - -] [+ - + -]
%e [- - + +] [- + - +]
%e [- - + +] [- - + +]
%Y Central coefficients of A333159.
%Y Cf. A333164, A333166.
%K nonn,more
%O 0,3
%A _Andrew Howroyd_, Mar 11 2020
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